context/core/netty: Add @CheckReturnValue to Context

[dapengzhang0]

This change is 

[carl-mastrangelo]

This needs to be check with an internal presubmit first.

[dapengzhang0]

This needs to be check with an internal presubmit first.

Yes, this change is scary.

[dapengzhang0]

The internal presubmit (CL/195275931) passed.

[carl-mastrangelo]

One other concern: this package has traditionally not had any dependencies (including DoNotMock). @adriancole I recall you being involved in the discussion around deps of Context. Would taking a compile time dependency on ErrorProne annotations be feasible?

[dapengzhang0]

One other concern: this package has traditionally not had any dependencies (including DoNotMock). @adriancole I recall you being involved in the discussion around deps of Context. Would taking a compile time dependency on ErrorProne annotations be feasible?

One of Context users, Zipkin instrumentation libs, has no deps, not even annotation jars. So this PR is deliberately not adding any dependencies. So com.google.errorprone.annotations.CheckReturnValue can not be used in io.grpc.Context. Instead, it uses io.grpc.Context.CheckReturnValue. It is up to the compiler, whether or not to apply the errorprone plug-in when compiling (in static analysis phase). Internally, the JavaBuilder will apply errorprone by default for all projects.

[carl-mastrangelo]

This probably deserves a comment why it isn't the error prone version.

[carl-mastrangelo]

one comment, but LGTM

[dapengzhang0]

Thanks for the review @carl-mastrangelo .