Make opencensus Stackdriver exporter respects initial_metadata option

[bianpengyuan]

Currently Opencensus tracer uses grpcService proto to configure its tracing client stub. It uses GoogleGrpcUtil to construct a channel and create client stub with that. However, the channel created there does not take care of initial_metadata from grpcService configure. This change fills in OCprepare_client_context option in export to make it respect initial metadata.

Signed-off-by: Pengyuan Bian bianpengyuan@google.com

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[asraa]

Thanks! Could you please add unit tests to cover this and for skipping the interceptor due to no initial metadata configured first?

[bianpengyuan]

@asraa I am having a hard time to find an easy way to test this. All current gRPC client tests are based on Envoy's async client/stream interface and implementation, but for this change we'd just want the test to verify the channel created could have initial metadata injected. Current options that I've thought about:

1. create a GoogleAsyncClientImpl with the initial metadata interceptor enabled channel, but leave initial_metadata in the configuration param, and verify the stream could still have initial metadata injected.
2. Enable mock interface generation for Greeter server and test the generated channel with mock grpc client and server.

Neither seems ideal to me.. any better ideas?

[asraa]

re-assigning to @htuch,who may be able to help a little more with testing.
I think maybe we should reduce the scope down to testing the written interceptor, but I'm uncertain how to do that right now.

[htuch]

@bianpengyuan a couple of things:

1. Can you explain in the commit message a bit more on the "what" vs. the motivation. I.e. we had some initial metadata plumbing before, what is the interceptor doing that provides value here? Ideally include links back to the Google gRPC docs of interest and how this would apply to end users.
2. We need some tests as discussed above. I think a good candidate place to put these would be https://github.com/envoyproxy/envoy/blob/master/test/common/grpc/google_async_client_impl_test.cc.

[bianpengyuan]

@htuch I updated the PR description. Yeah I have read through related tests. They are all based on AsyncClient / AsyncStream implementation and I don't find an easy way to apply them to this use case, which leads me to two options listed in #11831 (comment).

[htuch]

@bianpengyuan looking at the description, this makes me think that we're doing something pretty weird in OpenCensus. We already have another thread about OpenCensus doing its own thing with HTTP and curl (#11816), this is another place that it would be better for OpenCensus to provide a generic gRPC interface rather than relying on using Google gRPC libraries directly; when you do this you basically break Envoy's siloed threading model and lose observability via Envoy's normal mechanisms.

^^ is not actionable in this PR, but it would be good to make some progress here asynchronously, CC @g-easy @markdroth

For tests, I think you're going to need to do an integration test with OpenCensus and gRPC tracing enabled with a fake upstream (it can be Envoy's inbuilt FakeUpstream) that can validate the initial metadata received.

[bianpengyuan]

@htuch find an easier way to achieve this with the lastest opencensus. ptal, thanks!

[htuch]

LGTM, thanks!