Update gRPCContext wrapper class (#420)

There are a few cases where one needs to dig into `grpc.ServicerContext`
objects, and these fields were missing from our wrapper, which can cause
issues with implmementation.

CHANGELOG.md

@@ -15,6 +15,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   ([#387](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/387))
 - Update redis instrumentation to follow semantic conventions
   ([#403](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/403))
+- Update gRPC instrumentation to better wrap server context
+  ([#420](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/420))
 
 ### Added
 - `opentelemetry-instrumentation-urllib3` Add urllib3 instrumentation

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

@@ -71,6 +71,9 @@ def __init__(self, servicer_context, active_span):
         self.details = None
         super().__init__()
 
+    def __getattr__(self, attr):
+        return getattr(self._servicer_context, attr)
+
     def is_active(self, *args, **kwargs):
         return self._servicer_context.is_active(*args, **kwargs)