Add gRPC integration

[c24t]

Add a gRPC integration in the style of opencensus-ext-grpc, using gRPC python interceptors.

[codeboten]

The opentelemetry-auto-instr-python repo contains an integration for gRPC. We could port that into this repo.

[kapilt]

one request, please have a care for those that find grpc unwieldy as a large binary dependency, or complicated in its runtime usage analysis (considerable complexity in its implementation, background threads, locks), and make it optional instead of a require dependency of the opentelemetry sdk.

[update] i see all integrations are cleanly separated out into the ext packages, so above shouldn't be a concern.

[hectorhdzg]

OpenCensus implementation
https://github.com/census-instrumentation/opencensus-python/tree/master/contrib/opencensus-ext-grpc

OpenTracing implementation
https://github.com/opentracing-contrib/python-grpc

[andrewhsu]

Looks like related PR: #476

[c24t]

Fixed by #476.