Idea: support gRPC reflection

[esnible]

Requirement - what kind of business use case are you trying to solve?

I was running a Jaeger, but didn't know if it supported v3 Query API or not.

Problem - what in Jaeger blocks you from solving the requirement?

It is difficult to know the versions and APIs supported by the gRPC endpoint of a running Jaeger query instance.

Proposal - what do you suggest to solve the problem or improve the existing situation?

It takes one line of code to support list. See https://github.com/grpc/grpc-go/blob/master/Documentation/server-reflection-tutorial.md#enable-server-reflection

Any open questions to address

• Every service isn't required to support introspection. It is more of an aid for tools and troubleshooting.
• After rebuilding adding the line of code suggested above, grpcurl -plaintext localhost:16685 list works great! Unfortunately grpcurl -plaintext localhost:16685 describe doesn't, probably because of the reasons discussed at https://jbrandhorst.com/post/gogoproto/#reflection . It is unclear if it is worth the trouble needed to get describe working.

[yurishkuro]

+1

[pavolloffay]

I wonder if there is any perf impact, if there is we should probably enable it via a flag.

[esnible]

The performance impact should be close to zero. Adding reflection means increasing the gRPC method hash table from about 10 to about 12 entries and calling code from a module we already include if the gRPC method is "list" or "describe".