add trailing_metadata to _OpenTelemetryServicerContext

The trailing_metadata method was added to grpc._server.ServicerContext
in gRPC v1.38.0, and we would like to use it with OpenTelemetry in our
custom interceptors. Doing a hasattr check to maintain support for
earlier versions of gRPC.

Author: jbraswell

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

@@ -114,6 +114,13 @@ 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):
+        # This method was added to grpc.ServicerContext in gRPC v1.38.0. We do
+        # this hasattr check to maintain support for earlier gRPC versions.
+        if hasattr(self._servicer_context, "trailing_metadata"):
+            return self._servicer_context.trailing_metadata()
+        raise NotImplementedError()
+
     def abort(self, code, details):
         self.code = code
         self.details = details