fix(exporter-otlp-proto-http): log response body on metric export failure

CHANGELOG.md

@@ -12,6 +12,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+- `opentelemetry-exporter-otlp-proto-http`: include the response body (truncated to 1024 chars) in the metric exporter error log so collector-side rejection reasons are visible to users
+  ([#XXXX](https://github.com/open-telemetry/opentelemetry-python/pull/XXXX))
 - `opentelemetry-sdk`: add `additional_properties` support to generated config models via custom `datamodel-codegen` template, enabling plugin/custom component names to flow through typed dataclasses
   ([#5131](https://github.com/open-telemetry/opentelemetry-python/pull/5131))
 - Fix incorrect code example in `create_tracer()` docstring

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

@@ -116,6 +116,7 @@
 DEFAULT_METRICS_EXPORT_PATH = "v1/metrics"
 DEFAULT_TIMEOUT = 10  # in seconds
 _MAX_RETRYS = 6
+_MAX_LOGGED_BODY_CHARS = 1024
 
 
 class OTLPMetricExporter(MetricExporter, OTLPMetricExporterMixin):
@@ -292,16 +293,21 @@ def _export_with_retries(
                     export_error = error
                     retryable = isinstance(error, ConnectionError)
                     status_code = None
+                    body = None
                 else:
                     reason = resp.reason
                     retryable = _is_retryable(resp)
                     status_code = resp.status_code
+                    body = (resp.text or None) if not resp.ok else None
+                    if body is not None and len(body) > _MAX_LOGGED_BODY_CHARS:
+                        body = body[:_MAX_LOGGED_BODY_CHARS] + "...[truncated]"
 
                 if not retryable:
                     _logger.error(
-                        "Failed to export metrics batch code: %s, reason: %s",
+                        "Failed to export metrics batch code: %s, reason: %s, body: %s",
                         status_code,
                         reason,
+                        body,
                     )
                     error_attrs = (
                         {HTTP_RESPONSE_STATUS_CODE: status_code}
@@ -318,7 +324,8 @@ def _export_with_retries(
                 ):
                     _logger.error(
                         "Failed to export metrics batch due to timeout, "
-                        "max retries or shutdown."
+                        "max retries or shutdown. last response body: %s",
+                        body,
                     )
                     error_attrs = (
                         {HTTP_RESPONSE_STATUS_CODE: status_code}

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

@@ -433,6 +433,52 @@ def test_failure(self, mock_post):
             401,
         )
 
+    @patch.object(Session, "post")
+    def test_failure_logs_response_body(self, mock_post):
+        resp = Response()
+        resp.status_code = 400
+        resp.reason = "Bad Request"
+        resp._content = b"resource_metrics: data points exceed message size"
+        mock_post.return_value = resp
+
+        exporter = OTLPMetricExporter()
+
+        with self.assertLogs(level="ERROR") as logs:
+            self.assertEqual(
+                exporter.export(self.metrics["sum_int"]),
+                MetricExportResult.FAILURE,
+            )
+
+        self.assertTrue(
+            any(
+                "resource_metrics: data points exceed message size"
+                in record.getMessage()
+                for record in logs.records
+            ),
+            "Expected response body to appear in error log, "
+            f"got: {[r.getMessage() for r in logs.records]}",
+        )
+
+    @patch.object(Session, "post")
+    def test_failure_logs_truncates_long_response_body(self, mock_post):
+        resp = Response()
+        resp.status_code = 400
+        resp.reason = "Bad Request"
+        resp._content = b"x" * 5000
+        mock_post.return_value = resp
+
+        exporter = OTLPMetricExporter()
+
+        with self.assertLogs(level="ERROR") as logs:
+            self.assertEqual(
+                exporter.export(self.metrics["sum_int"]),
+                MetricExportResult.FAILURE,
+            )
+
+        joined = " ".join(record.getMessage() for record in logs.records)
+        self.assertIn("...[truncated]", joined)
+        self.assertNotIn("x" * 5000, joined)
+
     @patch.object(Session, "post")
     def test_serialization(self, mock_post):
         resp = Response()