Add registry keyword argument to PrometheusMetricReader

CHANGELOG.md

@@ -12,6 +12,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+- Add `registry` keyword argument to `PrometheusMetricReader` to allow passing a custom Prometheus registry
+  ([#5055](https://github.com/open-telemetry/opentelemetry-python/pull/5055))
+
 - Enabled the flake8-tidy-import plugins rules for the ruff linter. These rules throw warnings for relative imports in the modules. 
   ([#5019](https://github.com/open-telemetry/opentelemetry-python/pull/5019))
 - `opentelemetry-sdk`: Fix `AttributeError` in `ExplicitBucketHistogramAggregation` when applied to non-Histogram instruments without explicit boundaries

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

@@ -135,7 +135,10 @@ class PrometheusMetricReader(MetricReader):
     """Prometheus metric exporter for OpenTelemetry."""
 
     def __init__(
-        self, disable_target_info: bool = False, prefix: str = ""
+        self,
+        disable_target_info: bool = False,
+        prefix: str = "",
+        registry=REGISTRY,
     ) -> None:
         super().__init__(
             preferred_temporality={
@@ -151,7 +154,8 @@ def __init__(
         self._collector = _CustomCollector(
             disable_target_info=disable_target_info, prefix=prefix
         )
-        REGISTRY.register(self._collector)
+        self._registry = registry
+        self._registry.register(self._collector)
         self._collector._callback = self.collect
         self._prefix = prefix
 
@@ -166,7 +170,7 @@ def _receive_metrics(
         self._collector.add_metrics_data(metrics_data)
 
     def shutdown(self, timeout_millis: float = 30_000, **kwargs) -> None:
-        REGISTRY.unregister(self._collector)
+        self._registry.unregister(self._collector)
 
 
 class _CustomCollector:

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

@@ -16,7 +16,7 @@
 from unittest import TestCase
 from unittest.mock import Mock, patch
 
-from prometheus_client import generate_latest
+from prometheus_client import CollectorRegistry, generate_latest
 from prometheus_client.core import (
     CounterMetricFamily,
     GaugeMetricFamily,
@@ -55,6 +55,13 @@ def setUp(self):
             side_effect=self._mock_registry_register,
         )
 
+    def test_custom_registry(self):
+        custom_registry = CollectorRegistry()
+        reader = PrometheusMetricReader(registry=custom_registry)
+        # global REGISTRY should NOT be used
+        self._mock_registry_register.assert_not_called()
+        reader.shutdown()
+
     def verify_text_format(
         self, metric: Metric, expect_prometheus_text: str, prefix: str = ""
     ) -> None: