allow older grpc versions to raise AttributeError

Author: jbraswell

Diff for: instrumentation/opentelemetry-instrumentation-grpc/src/opentelemetry/instrumentation/grpc/_server.py

@@ -115,11 +115,7 @@ 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()
+        return self._servicer_context.trailing_metadata()
 
     def abort(self, code, details):
         self.code = code