improve grpc error stats

Title: Improve gRPC error stats

Description: Right now it appears that even gRPC responses with errors count toward the various 200/2xx stats counters, probably because the HTTP code itself will be 200.

The rq_success_ and rq_error_ stats take into account gRPC errors, but only if the gRPC error falls in the 5xx range:

      absl::optional<Grpc::Status::GrpcStatus> grpc_status = Grpc::Common::getGrpcStatus(headers);
      if (grpc_status &&
          !Http::CodeUtility::is5xx(Grpc::Utility::grpcToHttpStatus(grpc_status.value()))) {
        upstream_request_->upstream_host_->stats().rq_success_.inc();
      } else {
        upstream_request_->upstream_host_->stats().rq_error_.inc();
      }

It would be nice to add options to Envoy to:

1. count gRPC codes toward the existing http-style stats and/or
2. emit new metrics for gRPC codes (i.e. 0-16)