bugfix(exporter): set user agent for otlp metrics exporter

CHANGELOG.md

@@ -7,6 +7,9 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+- Fix user agent in OTLP HTTP metrics exporter
+  ([#4475](https://github.com/open-telemetry/opentelemetry-python/pull/4475))
+
 ## Version 1.31.0/0.52b0 (2025-03-12)
 
 - semantic-conventions: Bump to 1.31.0

exporter/opentelemetry-exporter-otlp-proto-http/src/opentelemetry/exporter/otlp/proto/http/metric_exporter/__init__.py

@@ -40,7 +40,10 @@
 from opentelemetry.exporter.otlp.proto.common.metrics_encoder import (
     encode_metrics,
 )
-from opentelemetry.exporter.otlp.proto.http import Compression
+from opentelemetry.exporter.otlp.proto.http import (
+    _OTLP_HTTP_HEADERS,
+    Compression,
+)
 from opentelemetry.proto.collector.metrics.v1.metrics_service_pb2 import (  # noqa: F401
     ExportMetricsServiceRequest,
 )
@@ -152,9 +155,7 @@ def __init__(
         self._compression = compression or _compression_from_env()
         self._session = session or requests.Session()
         self._session.headers.update(self._headers)
-        self._session.headers.update(
-            {"Content-Type": "application/x-protobuf"}
-        )
+        self._session.headers.update(_OTLP_HTTP_HEADERS)
         if self._compression is not Compression.NoCompression:
             self._session.headers.update(
                 {"Content-Encoding": self._compression.value}

exporter/opentelemetry-exporter-otlp-proto-http/tests/metrics/test_otlp_metrics_exporter.py

@@ -32,6 +32,7 @@
     DEFAULT_TIMEOUT,
     OTLPMetricExporter,
 )
+from opentelemetry.exporter.otlp.proto.http.version import __version__
 from opentelemetry.sdk.environment_variables import (
     OTEL_EXPORTER_OTLP_CERTIFICATE,
     OTEL_EXPORTER_OTLP_CLIENT_CERTIFICATE,
@@ -124,6 +125,15 @@ def test_constructor_default(self):
         self.assertIs(exporter._compression, DEFAULT_COMPRESSION)
         self.assertEqual(exporter._headers, {})
         self.assertIsInstance(exporter._session, Session)
+        self.assertIn("User-Agent", exporter._session.headers)
+        self.assertEqual(
+            exporter._session.headers.get("Content-Type"),
+            "application/x-protobuf",
+        )
+        self.assertEqual(
+            exporter._session.headers.get("User-Agent"),
+            "OTel-OTLP-Exporter-Python/" + __version__,
+        )
 
     @patch.dict(
         "os.environ",