Add stack trace on StatusCode.UNKNOWN export error

A deployment I ran over the weekend is logging the following message
hundreds of times per minute:

> "Failed to export metrics to otlp.eu01.nr-data.net, error code: StatusCode.UNKNOWN"

For this status code - 'Unknown' - it would be nice to have the full
error logged, as the message above is very hard to debug.

Author: jakewins

exporter/opentelemetry-exporter-otlp-proto-grpc/src/opentelemetry/exporter/otlp/proto/grpc/exporter.py

@@ -308,6 +308,7 @@ def _export(
                             self._exporting,
                             self._endpoint,
                             error.code(),
+                            exc_info=error.code() == StatusCode.UNKNOWN,
                         )
 
                     if error.code() == StatusCode.OK:

exporter/opentelemetry-exporter-otlp-proto-grpc/tests/test_otlp_metrics_exporter.py

@@ -112,6 +112,14 @@ def Export(self, request, context):
         return ExportMetricsServiceResponse()
 
 
+class MetricsServiceServicerUNKNOWN(MetricsServiceServicer):
+    # pylint: disable=invalid-name,unused-argument,no-self-use
+    def Export(self, request, context):
+        context.set_code(StatusCode.UNKNOWN)
+
+        return ExportMetricsServiceResponse()
+
+
 class MetricsServiceServicerSUCCESS(MetricsServiceServicer):
     # pylint: disable=invalid-name,unused-argument,no-self-use
     def Export(self, request, context):
@@ -441,6 +449,31 @@ def test_unavailable_delay(self, mock_sleep, mock_expo):
         )
         mock_sleep.assert_called_with(4)
 
+    @patch(
+        "opentelemetry.exporter.otlp.proto.grpc.exporter._create_exp_backoff_generator"
+    )
+    @patch("opentelemetry.exporter.otlp.proto.grpc.exporter.sleep")
+    @patch("opentelemetry.exporter.otlp.proto.grpc.exporter.logger")
+    def test_unknown_logs(self, mock_sleep, mock_expo, mock_logger):
+
+        mock_expo.configure_mock(**{"return_value": [1]})
+
+        add_MetricsServiceServicer_to_server(
+            MetricsServiceServicerUNKNOWN(), self.server
+        )
+        self.assertEqual(
+            self.exporter.export(self.metrics["sum_int"]),
+            MetricExportResult.FAILURE,
+        )
+        mock_sleep.assert_called_with(1)
+        mock_logger.error.assert_called_with(
+            "Failed to export %s to %s, error code: %s",
+            "metrics",
+            "TODO",
+            StatusCode.UNKNOWN,
+            exc_info=True,
+        )
+
     def test_success(self):
         add_MetricsServiceServicer_to_server(
             MetricsServiceServicerSUCCESS(), self.server