Add custom label for per-RPC metrics

[ejona86]

TODO: Add tests

I've created this so I can reference it as an example from gRFC A108.

[ejona86]

@kannanjgithub, I added you as you may want to pre-review, before the gRFC is finalized. I'm really only expecting bikeshedding on the label name (as part of the gRFC). This is introducing a new stable API, so take a special look at the Grpc class.

[kannanjgithub]

Thanks for putting this up. Changes look good.