Resolve #72 - gRPC Interceptor

[reicheltp]

Resolves #72 by introducing client and server interceptors. It automatically creates spans and messages respecting the specification defined here: https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/data-rpc.md

The interceptors can simply be used during grpc.Dial and grpc.NewServer.

Client Interceptor

tracer := global.Tracer("client-tracer")

var conn *grpc.ClientConn
conn, err := grpc.Dial(":7777", grpc.WithInsecure(),
	grpc.WithUnaryInterceptor(grpctrace.UnaryClientInterceptor(tracer)),
	grpc.WithStreamInterceptor(grpctrace.StreamClientInterceptor(tracer)),
)

Server Interceptor

tracer := global.Tracer("server-tracer")

s := grpc.NewServer(
	grpc.UnaryInterceptor(grpctrace.UnaryServerInterceptor(tracer)),
	grpc.StreamInterceptor(grpctrace.StreamServerInterceptor(tracer)),
)

Known Issues

Currently the message.compressed_size is not logged as we found no way to gather this information from a proto.Message.

[krnowak]

Please sign the CLA and please fix the CI issues (run make on your host and include the changes generated by linters and go mod tidy in the PR). Thanks!

[lizthegrey]

CLA still doesn't appear to be signed.

[reicheltp]

@lizthegrey Sorry about the CLA. It's on the desk of my manager. I hope he gets this done during the next couple of days.

[rghetia]

@reicheltp any update on the CLA?

[reicheltp]

@rghetia I've got good news: The CCLA is signed.

[jmacd]

We have a flaky test.

[lizthegrey]

Forcing a test rerun.

[jmacd]

For some reason I've lost the ability to update this branch. (Did a GH setting change?) Will you update this branch so I can merge it? Thanks.