add trailing_metadata to _OpenTelemetryServicerContext (#871)

The trailing_metadata method was added to grpc._server.ServicerContext
in gRPC v1.38.0.

Author: jbraswell

CHANGELOG.md

@@ -9,6 +9,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- `opentelemetry-instrumentation-grpc` added `trailing_metadata` to _OpenTelemetryServicerContext.
+  ([#871](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/871))
 - `opentelemetry-instrumentation-asgi` now returns a `traceresponse` response header.
   ([#817](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/817))
 - `opentelemetry-instrumentation-kafka-python` added kafka-python module instrumentation.

instrumentation/opentelemetry-instrumentation-grpc/src/opentelemetry/instrumentation/grpc/_server.py

@@ -114,6 +114,9 @@ def send_initial_metadata(self, *args, **kwargs):
     def set_trailing_metadata(self, *args, **kwargs):
         return self._servicer_context.set_trailing_metadata(*args, **kwargs)
 
+    def trailing_metadata(self):
+        return self._servicer_context.trailing_metadata()
+
     def abort(self, code, details):
         self.code = code
         self.details = details