support prefix for prometheus exporter (open-telemetry#4895)

* support prefix for prometheus exporter

* update changelog

* fix parameter order

* precommit

* ci

---------

Co-authored-by: Emídio Neto <9735060+emdneto@users.noreply.github.com>

Author: jpb

CHANGELOG.md

@@ -22,6 +22,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   ([#4935](https://github.com/open-telemetry/opentelemetry-python/pull/4935))
 - `opentelemetry-sdk`: upgrade vendored OTel configuration schema from v1.0.0-rc.3 to v1.0.0
   ([#4965](https://github.com/open-telemetry/opentelemetry-python/pull/4965))
+- `opentelemetry-exporter-prometheus`: Fix metric name prefix
+  ([#4895](https://github.com/open-telemetry/opentelemetry-python/pull/4895))
 
 ## Version 1.40.0/0.61b0 (2026-03-04)
 

docs/examples/metrics/prometheus-grafana/prometheus-monitor.py

@@ -11,7 +11,7 @@
 start_http_server(port=8000, addr="localhost")
 # Exporter to export metrics to Prometheus
 prefix = "MyAppPrefix"
-reader = PrometheusMetricReader(prefix)
+reader = PrometheusMetricReader(prefix=prefix)
 # Meter is responsible for creating and recording metrics
 set_meter_provider(MeterProvider(metric_readers=[reader]))
 meter = get_meter_provider().get_meter("view-name-change", "0.1.2")

exporter/opentelemetry-exporter-prometheus/src/opentelemetry/exporter/prometheus/__init__.py

@@ -38,7 +38,7 @@
 
     # Exporter to export metrics to Prometheus
     prefix = "MyAppPrefix"
-    reader = PrometheusMetricReader(prefix)
+    reader = PrometheusMetricReader(prefix=prefix)
 
     # Meter is responsible for creating and recording metrics
     set_meter_provider(MeterProvider(metric_readers=[reader]))
@@ -131,7 +131,9 @@ def _convert_buckets(
 class PrometheusMetricReader(MetricReader):
     """Prometheus metric exporter for OpenTelemetry."""
 
-    def __init__(self, disable_target_info: bool = False) -> None:
+    def __init__(
+        self, disable_target_info: bool = False, prefix: str = ""
+    ) -> None:
         super().__init__(
             preferred_temporality={
                 Counter: AggregationTemporality.CUMULATIVE,
@@ -142,9 +144,12 @@ def __init__(self, disable_target_info: bool = False) -> None:
                 ObservableGauge: AggregationTemporality.CUMULATIVE,
             }
         )
-        self._collector = _CustomCollector(disable_target_info)
+        self._collector = _CustomCollector(
+            disable_target_info=disable_target_info, prefix=prefix
+        )
         REGISTRY.register(self._collector)
         self._collector._callback = self.collect
+        self._prefix = prefix
 
     def _receive_metrics(
         self,
@@ -167,11 +172,12 @@ class _CustomCollector:
     https://github.com/prometheus/client_python#custom-collectors
     """
 
-    def __init__(self, disable_target_info: bool = False):
+    def __init__(self, disable_target_info: bool = False, prefix: str = ""):
         self._callback = None
         self._metrics_datas: Deque[MetricsData] = deque()
         self._disable_target_info = disable_target_info
         self._target_info = None
+        self._prefix = prefix
 
     def add_metrics_data(self, metrics_data: MetricsData) -> None:
         """Add metrics to Prometheus data"""
@@ -227,7 +233,10 @@ def _translate_to_prometheus(
             label_values_data_points = []
             values = []
 
-            metric_name = sanitize_full_name(metric.name)
+            metric_name = metric.name
+            if self._prefix:
+                metric_name = self._prefix + "_" + metric_name
+            metric_name = sanitize_full_name(metric_name)
             metric_description = metric.description or ""
             metric_unit = map_unit(metric.unit)
 

exporter/opentelemetry-exporter-prometheus/tests/test_prometheus_exporter.py

@@ -55,7 +55,7 @@ def setUp(self):
         )
 
     def verify_text_format(
-        self, metric: Metric, expect_prometheus_text: str
+        self, metric: Metric, expect_prometheus_text: str, prefix: str = ""
     ) -> None:
         metrics_data = MetricsData(
             resource_metrics=[
@@ -73,7 +73,7 @@ def verify_text_format(
             ]
         )
 
-        collector = _CustomCollector(disable_target_info=True)
+        collector = _CustomCollector(disable_target_info=True, prefix=prefix)
         collector.add_metrics_data(metrics_data)
         result_bytes = generate_latest(collector)
         result = result_bytes.decode("utf-8")
@@ -463,6 +463,30 @@ def test_metric_name(self):
                 """
             ),
         )
+        self.verify_text_format(
+            _generate_sum(name="test_counter_w_prefix", value=1, unit=""),
+            dedent(
+                """\
+                # HELP foo_test_counter_w_prefix_total foo
+                # TYPE foo_test_counter_w_prefix_total counter
+                foo_test_counter_w_prefix_total{a="1",b="true"} 1.0
+                """
+            ),
+            prefix="foo",
+        )
+        self.verify_text_format(
+            _generate_sum(
+                name="test_counter_w_invalid_chars_prefix", value=1, unit=""
+            ),
+            dedent(
+                """\
+                # HELP _foo_test_counter_w_invalid_chars_prefix_total foo
+                # TYPE _foo_test_counter_w_invalid_chars_prefix_total counter
+                _foo_test_counter_w_invalid_chars_prefix_total{a="1",b="true"} 1.0
+                """
+            ),
+            prefix="#foo",
+        )
         self.verify_text_format(
             _generate_sum(name="1leading_digit", value=1, unit=""),
             dedent(