Fix OTLP metric/log protocol settings to fallback correctly

Author: bouwkast

tracer/src/Datadog.Trace/Configuration/TracerSettings.cs

@@ -250,10 +250,17 @@ not null when string.Equals(x, "http/protobuf", StringComparison.OrdinalIgnoreCa
                                     },
                                     validator: null);
 
+            var otlpGeneralProtocolName = OtlpGeneralProtocol switch
+            {
+                OtlpProtocol.Grpc => "grpc",
+                OtlpProtocol.HttpProtobuf => "http/protobuf",
+                _ => "grpc",
+            };
+
             OtlpMetricsProtocol = config
                                  .WithKeys(ConfigurationKeys.OpenTelemetry.ExporterOtlpMetricsProtocol)
                                  .GetAs(
-                                      defaultValue: new(OtlpProtocol.Grpc, "grpc"),
+                                      defaultValue: new(OtlpGeneralProtocol, otlpGeneralProtocolName),
                                       converter: x => x switch
                                       {
                                           not null when string.Equals(x, "grpc", StringComparison.OrdinalIgnoreCase) => OtlpProtocol.Grpc,
@@ -312,7 +319,7 @@ not null when string.Equals(x, "lowmemory", StringComparison.OrdinalIgnoreCase)
             OtlpLogsProtocol = config
                              .WithKeys(ConfigurationKeys.OpenTelemetry.ExporterOtlpLogsProtocol)
                              .GetAs(
-                                  defaultValue: new(OtlpProtocol.Grpc, "grpc"),
+                                  defaultValue: new(OtlpGeneralProtocol, otlpGeneralProtocolName),
                                   converter: x => x switch
                                   {
                                       not null when string.Equals(x, "grpc", StringComparison.OrdinalIgnoreCase) => OtlpProtocol.Grpc,

tracer/test/Datadog.Trace.Tests/Configuration/TracerSettingsTests.cs

@@ -978,6 +978,8 @@ public void OtelMetricExportTimeoutCustomValues(string value, int expected)
         [InlineData("invalid", null, OtlpProtocol.Grpc)]
         [InlineData("http/protobuf", null, OtlpProtocol.HttpProtobuf)]
         [InlineData("grpc", "http/protobuf", OtlpProtocol.Grpc)]
+        [InlineData(null, "http/protobuf", OtlpProtocol.HttpProtobuf)]
+        [InlineData(null, "grpc", OtlpProtocol.Grpc)]
         public void OtlpProtocolFallbacks(string metricsProtocol, string generalProtocol, object expected)
         {
             var source = CreateConfigurationSource(
@@ -1073,6 +1075,8 @@ public void PartialFlushMinSpans(string value, int expected)
         [InlineData("invalid", null, OtlpProtocol.Grpc)]
         [InlineData("http/protobuf", null, OtlpProtocol.HttpProtobuf)]
         [InlineData("grpc", "http/protobuf", OtlpProtocol.Grpc)]
+        [InlineData(null, "http/protobuf", OtlpProtocol.HttpProtobuf)]
+        [InlineData(null, "grpc", OtlpProtocol.Grpc)]
         public void OtlpLogsProtocolFallbacks(string logsProtocol, string generalProtocol, object expected)
         {
             var source = CreateConfigurationSource(