apply gRPC tags and take a sampling decision on Akka gRPC server requests, fixes #1118

[ivantopo]

Adds a few rpc tags and ensures that Kamon will take a sampling decision on matched gRPC services. I'm not including tests because Akka gRPC doesn't work on Scaka 2.11 and the entire process of tweaking the build would be considerable more work than the tiny instrumentation addition.

[jtjeferreira]

LGTM, but would be nice to have some tests

[ivantopo]

Yeah, I'm considering creating a new subproject kamon-akka-grpc and putting it all there, only for scala 2.12+. I was also looking at the client side of it and I think it wouldn't be super complicated to implement. If I find time for it over the weekend I'll put it all together, otherwise we ship only the server side and do the subproject+client later

[ivantopo]

Hey @jtjeferreira, I started working on separating the akka-grpc instrumentation into its own project with tests and everything, but then it breaks the build because the Play gRCP support (used in kamon-play) uses the gRPC SBT plugin version 1.x, but the one for Akka HTTP uses 2.1.3 😭

Maybe this can be worked around by making a jar with all the play-specific code and adding it as a dependency, so the build stays with the latest plugin version for akka-grpc. I'll probably get back to this later today or tomorrow.

[ivantopo]

Turns out that the Akka gRPC instrumentation covers what gets generated by play-grpc, so I'm removing everything grpc-related from the Play instrumentation and let the akka-grpc instrumentation do all the work