gRPC AttributeExtractors should have access to gRPC Metadata passed to TracingClientCall.start()

[evantorrie]

Is your feature request related to a problem? Please describe.

TL;DR; Custom AttributeExtractors added to instrumented gRPC client calls lack access to gRPC metadata sent in call.

Currently, the gRPC instrumentation constructs a GrpcRequest object at the beginning of interceptCall() at which point the gRPC Metadata is unset - set to null in the code.

gRPC Metadata for outgoing client calls is available at the point of TracingClientCall.start(Listener<RESPONSE> responseListener, Metadata header). However, the gRPC request object which is passed to instrumenter.end() never has its corresponding Metadata private field updated.

Describe the solution you'd like

instrumenter.end() calls each AttributeExtractor's onEnd() method. The request object passed to these methods should include an updated version of the metadata sourced from the metadata parameter passed to TracingClientCall.start().

Describe alternatives you've considered

Duplicating most of the code from the grpc Instrumentation simply to ensure that the GrpcRequest object is updated with the appropriate metadata to make it available to our own AttributeExtractors.

Additional context

This issue was raised at the May 10th Java SIG.

[trask]

hi @evantorrie! it would be great if you are able to send a PR for this

[laurit]

Resolved with #6125