Emit useful gRPC result metrics

[E. Wolak]

Misk's default metrics, namely http_request_latency_ms (by action, caller, status code) are HTTP-centric. For gRPC responses, we should probably emit some gRPC-specific metrics, especially because HTTP status code in gRPC is strange (see #1983). https://github.com/grpc-ecosystem/java-grpc-prometheus might be a good place to start.