Integrate with sentry

[cdevienne]

When implementing panics handling, it would be nice to have the option to report them to a sentry instance.

[mdevan]

The user services built using nRPC could use this too. Also a few other such "core" services:

• logging - nats.Publish() your logs to a "log" service
• application metrics - publish your metrics, or discover pull endpoints for them
• tracing - be able to tag the reported logs or panics or metrics with a request tracking ID

Basically, my point is that there are a set of few "core" services that are must-have/useful for anyone using nRPC -- if we don't provide them, the users will have to invent it themselves. We should structure these as a "nrpc-core" package which includes .proto, generated clients and simple/standard server implementations.