[Exporter.Prometheus] Add target_info fallback

src/OpenTelemetry.Exporter.Prometheus.AspNetCore/CHANGELOG.md

@@ -66,6 +66,9 @@ Notes](../../RELEASENOTES.md).
   `X-Prometheus-Scrape-Timeout-Seconds` HTTP request header.
   ([#7252](https://github.com/open-telemetry/opentelemetry-dotnet/issues/7252))
 
+* Add Prometheus text fallback `target_info` output as a gauge.
+  ([#7238](https://github.com/open-telemetry/opentelemetry-dotnet/pull/7238))
+
 ## 1.15.3-beta.1
 
 Released 2026-Apr-21

src/OpenTelemetry.Exporter.Prometheus.HttpListener/CHANGELOG.md

@@ -81,6 +81,9 @@ Notes](../../RELEASENOTES.md).
   `X-Prometheus-Scrape-Timeout-Seconds` HTTP request header.
   ([#7252](https://github.com/open-telemetry/opentelemetry-dotnet/issues/7252))
 
+* Add Prometheus text fallback `target_info` output as a gauge.
+  ([#7238](https://github.com/open-telemetry/opentelemetry-dotnet/pull/7238))
+
 ## 1.15.3-beta.1
 
 Released 2026-Apr-21

src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusCollectionManager.cs

@@ -20,7 +20,8 @@ internal sealed class PrometheusCollectionManager
     private int metricsCacheCount;
     private byte[] plainTextBuffer = new byte[85000]; // encourage the object to live in LOH (large object heap)
     private byte[] openMetricsBuffer = new byte[85000]; // encourage the object to live in LOH (large object heap)
-    private int targetInfoBufferLength = -1; // zero or positive when target_info has been written for the first time
+    private int plainTextTargetInfoBufferLength = -1;
+    private int openMetricsTargetInfoBufferLength = -1;
     private ArraySegment<byte> previousPlainTextDataView;
     private ArraySegment<byte> previousOpenMetricsDataView;
     private int globalLockState;
@@ -237,10 +238,10 @@ private ExportResult OnCollect(in Batch<Metric> metrics)
 
         try
         {
+            cursor = this.WriteTargetInfo(ref buffer);
+
             if (this.exporter.OpenMetricsRequested)
             {
-                cursor = this.WriteTargetInfo(ref buffer);
-
                 this.scopes.Clear();
 
                 foreach (var metric in metrics)
@@ -356,14 +357,17 @@ private ExportResult OnCollect(in Batch<Metric> metrics)
 
     private int WriteTargetInfo(ref byte[] buffer)
     {
-        if (this.targetInfoBufferLength < 0)
+        ref var targetInfoBufferLength = ref this.exporter.OpenMetricsRequested
+            ? ref this.openMetricsTargetInfoBufferLength
+            : ref this.plainTextTargetInfoBufferLength;
+
+        if (targetInfoBufferLength < 0)
         {
             while (true)
             {
                 try
                 {
-                    this.targetInfoBufferLength = PrometheusSerializer.WriteTargetInfo(buffer, 0, this.exporter.Resource, openMetricsRequested: true);
-
+                    targetInfoBufferLength = PrometheusSerializer.WriteTargetInfo(buffer, 0, this.exporter.Resource, this.exporter.OpenMetricsRequested);
                     break;
                 }
                 catch (IndexOutOfRangeException)
@@ -376,7 +380,7 @@ private int WriteTargetInfo(ref byte[] buffer)
             }
         }
 
-        return this.targetInfoBufferLength;
+        return targetInfoBufferLength;
     }
 
     private PrometheusMetric GetPrometheusMetric(Metric metric)

src/OpenTelemetry.Exporter.Prometheus.HttpListener/Internal/PrometheusSerializer.cs

@@ -500,21 +500,37 @@ public static int WriteTargetInfo(byte[] buffer, int cursor, Resource resource,
             return cursor;
         }
 
-        cursor = WriteAsciiStringNoEscape(buffer, cursor, "# TYPE target info");
+        using var attributes = resource.Attributes.GetEnumerator();
+        if (!attributes.MoveNext())
+        {
+            return cursor;
+        }
+
+        // "If info-typed metric families are not yet supported...a gauge-typed metric
+        // family named target_info with a constant value of 1 MUST be used instead.".
+        // See https://opentelemetry.io/docs/specs/otel/compatibility/prometheus_and_openmetrics/#resource-attributes-1
+        cursor = WriteAsciiStringNoEscape(buffer, cursor, "# TYPE ");
+        cursor = WriteAsciiStringNoEscape(buffer, cursor, openMetricsRequested ? "target" : "target_info");
+        buffer[cursor++] = unchecked((byte)' ');
+        cursor = WriteAsciiStringNoEscape(buffer, cursor, openMetricsRequested ? "info" : "gauge");
         buffer[cursor++] = ASCII_LINEFEED;
 
-        cursor = WriteAsciiStringNoEscape(buffer, cursor, "# HELP target Target metadata");
+        cursor = WriteAsciiStringNoEscape(buffer, cursor, "# HELP ");
+        cursor = WriteAsciiStringNoEscape(buffer, cursor, openMetricsRequested ? "target" : "target_info");
+        cursor = WriteAsciiStringNoEscape(buffer, cursor, " Target metadata");
         buffer[cursor++] = ASCII_LINEFEED;
 
         cursor = WriteAsciiStringNoEscape(buffer, cursor, "target_info");
         buffer[cursor++] = unchecked((byte)'{');
 
-        foreach (var attribute in resource.Attributes)
+        do
         {
+            var attribute = attributes.Current;
             cursor = WriteLabel(buffer, cursor, attribute.Key, attribute.Value, openMetricsRequested);
 
             buffer[cursor++] = unchecked((byte)',');
         }
+        while (attributes.MoveNext());
 
         cursor--; // Write over the last written comma
 

test/OpenTelemetry.Exporter.Prometheus.AspNetCore.Tests/PrometheusExporterMiddlewareTests.cs

@@ -573,6 +573,9 @@ private static async Task VerifyAsync(
 
                     """.ReplaceLineEndings()
             : $$"""
+                    # TYPE target_info gauge
+                    # HELP target_info Target metadata
+                    target_info{service_name="my_service",service_instance_id="id1"} 1
                     # TYPE counter_double_bytes_total counter
                     # UNIT counter_double_bytes_total bytes
                     counter_double_bytes_total{otel_scope_name="{{MeterName}}",otel_scope_version="{{MeterVersion}}",{{additionalTags}}key1="value1",key2="value2"} 101.17

test/OpenTelemetry.Exporter.Prometheus.AspNetCore.Tests/PrometheusIntegrationTests.cs

@@ -99,34 +99,38 @@ public async Task Prometheus_Can_Scrape_Metrics(string scrapeProtocol) => await
 
             await WaitForServiceDiscoveryAsync(prometheusBaseAddress, outputHelper, cts.Token);
 
-            IReadOnlyList<string> series = [];
+            HashSet<string> expectedSeries =
+            [
+#if NET10_0_OR_GREATER
+                "aspnetcore_memory_pool_allocated_bytes_total",
+#endif
+                "http_server_active_requests",
+                "http_server_request_duration_seconds_bucket",
+                "http_server_request_duration_seconds_count",
+                "http_server_request_duration_seconds_sum",
+                "kestrel_active_connections",
+                "kestrel_connection_duration_seconds_bucket",
+                "kestrel_connection_duration_seconds_count",
+                "kestrel_connection_duration_seconds_sum",
+                "processed_bytes_total",
+                "queue_balance",
+                "temperature_celsius",
+            ];
+
+            HashSet<string> actualSeries = [];
 
             // Assert
             while (!cts.IsCancellationRequested)
             {
-                series = await WaitForMetricsSeriesAsync(prometheusBaseAddress, outputHelper, cts.Token);
+                actualSeries = await WaitForMetricsSeriesAsync(prometheusBaseAddress, outputHelper, cts.Token);
 
-                if (series.Contains("temperature_celsius"))
+                if (actualSeries.IsProperSupersetOf(expectedSeries))
                 {
                     break;
                 }
             }
 
-            Assert.Contains("http_server_active_requests", series);
-            Assert.Contains("http_server_request_duration_seconds_bucket", series);
-            Assert.Contains("http_server_request_duration_seconds_count", series);
-            Assert.Contains("http_server_request_duration_seconds_sum", series);
-            Assert.Contains("kestrel_active_connections", series);
-            Assert.Contains("kestrel_connection_duration_seconds_bucket", series);
-            Assert.Contains("kestrel_connection_duration_seconds_count", series);
-            Assert.Contains("kestrel_connection_duration_seconds_sum", series);
-            Assert.Contains("processed_bytes_total", series);
-            Assert.Contains("queue_balance", series);
-            Assert.Contains("temperature_celsius", series);
-
-#if NET10_0_OR_GREATER
-            Assert.Contains("aspnetcore_memory_pool_allocated_bytes_total", series);
-#endif
+            Assert.ProperSuperset(expectedSeries, actualSeries);
         }
         finally
         {
@@ -138,7 +142,7 @@ public async Task Prometheus_Can_Scrape_Metrics(string scrapeProtocol) => await
             await prometheus.DisposeAsync();
         }
 
-        static async Task<IReadOnlyList<string>> WaitForMetricsSeriesAsync(
+        static async Task<HashSet<string>> WaitForMetricsSeriesAsync(
             Uri baseAddress,
             ITestOutputHelper outputHelper,
             CancellationToken cancellationToken)
@@ -182,7 +186,7 @@ static async Task<IReadOnlyList<string>> WaitForMetricsSeriesAsync(
                                 }
                             }
 
-                            return [.. series];
+                            return series;
                         }
                     }
                 }

test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/PrometheusHttpListenerTests.cs

@@ -628,7 +628,10 @@ private static async Task RunPrometheusExporterHttpServerIntegrationTest(
                   + "# UNIT counter_double_bytes bytes\n"
                   + $"counter_double_bytes_total{{otel_scope_name='{MeterName}',otel_scope_version='{MeterVersion}',{additionalTags}key1='value1',key2='value2'}} 101.17\n"
                   + "# EOF\n"
-                : "# TYPE counter_double_bytes_total counter\n"
+                : "# TYPE target_info gauge\n"
+                  + "# HELP target_info Target metadata\n"
+                  + "target_info{service_name='my_service',service_instance_id='id1'} 1\n"
+                  + "# TYPE counter_double_bytes_total counter\n"
                   + "# UNIT counter_double_bytes_total bytes\n"
                   + $"counter_double_bytes_total{{otel_scope_name='{MeterName}',otel_scope_version='{MeterVersion}',{additionalTags}key1='value1',key2='value2'}} 101.17\n"
                   + "# EOF\n";

test/OpenTelemetry.Exporter.Prometheus.HttpListener.Tests/PrometheusSerializerTests.cs

@@ -6,6 +6,7 @@
 using System.Globalization;
 using System.Text;
 using OpenTelemetry.Metrics;
+using OpenTelemetry.Resources;
 using OpenTelemetry.Tests;
 using Xunit;
 
@@ -1141,6 +1142,19 @@ public void WriteDoubleFormatsNaN()
         Assert.Equal("NaN", Encoding.UTF8.GetString(buffer, 0, cursor));
     }
 
+    [Theory]
+    [InlineData(false)]
+    [InlineData(true)]
+    public void WriteTargetInfoSkipsEmptyNonSingletonResource(bool openMetricsRequested)
+    {
+        var buffer = new byte[128];
+        var resource = Resource.Empty.Merge(Resource.Empty);
+
+        var cursor = PrometheusSerializer.WriteTargetInfo(buffer, 0, resource, openMetricsRequested);
+
+        Assert.Equal(0, cursor);
+    }
+
     [Fact]
     public void WriteUnicodeStringEncodesSurrogatePairsAsUtf8ScalarValues()
     {