feat: Use grpc.WithStatsHandler to record gRPC metrics

[bryanhuhta]

This implements a basic stats.Handler to manage recording metrics for gRPC clients and servers. This is an improvement (possible minor) over using an interceptor because it will more precisely report timings and byte sizes as it gets execute outside of the interceptor evaluation chain.

I plan to experiment with using this over interceptors to see if there's any tangible difference in measurements (both in accuracy and performance).

I have not tested this so I cannot vouch for its accuracy yet.