Enable tracing for the generated clients

[aperkins1310]

As the client api is currently netty based, we don't have access to the Akka HttpRequests and HttpResponses in the same way as we do for servers. This means we cannot add tracing info to the requests. Thankfully grpc-java and the netty client has OpenCensus tracing integration, it is just not enabled for the clients generated here.

Its a simple one liner to enable:
.setSampledToLocalTracing(true)

This would allow adding a module for OpenCensus Scala for akka-grpc, for tracing (and eventually Stats/ metrics as well)

Is this okay to enable by default for generated clients?

Will add a PR for this and see what the consensus is.