This is a small refactor to how opencensus metrics are reported when
receiving responses from the server.

There's effectively two cases where we consider an error to have
occurred on receive:

* Invoking gRPC Recv() on the connection emitted an error, typically a
  transport issue

* The service embedded an error in the response, which is more akin to
  an application error (failed to commit data, offset mismatch, etc).

This CL ensures we increment the existing AppendResponseError count
metric in both causes, and deals with the unlikely scenario where we're
unable to tag the report with the status code.  When that happens, we
simply record the metric uncoded.