Micrometer: Support gRPC monitoring

[jacobk]

It would be great with auto configured monitoring for:

• gRPC client requests using generated Java code
• gRPC server handlers based on e.g. https://github.com/LogNet/grpc-spring-boot-starter

[jkschneider]

Some prior art here: https://github.com/grpc-ecosystem/java-grpc-prometheus

[ST-DDT]

There is also an interceptor based implementation available in https://github.com/yidongnan/grpc-spring-boot-starter:

• shared
• client
• server

Feel free to move this upstream. I can create a PR with the required classes, just tell me where to add them to.

EDIT: I plan to add support for netty's ELGs and executors too (either here or at yidongnan's repo)

[racevedoo]

Any work here guys? @ST-DDT did you start doing something? I was thinking of creating a PR

[ST-DDT]

No, IMO configuring support for monitoring netty's ELGs is way more complicated then I originally anticipated, so I dropped it for now. (Especially how to restore the grpc's default settings while still retaining maintainability and configurability).
One of the biggest issues there (if you dive for netty metrics), is that grpc shades netty.

Feel free to reuse the metric collection code linked above. Just ping me if you create the PR.
If you feel uncomfortable with copy, pasting someone else's code, I could create the PR for the basic metrics for you.

[racevedoo]

@ST-DDT I guess we could start with the basic metrics and then improve to include netty metrics later. The reason for this is that, for some use cases, basic metrics are good enough.

WDYT?

[ST-DDT]

Finally I had time to create a PR to add the metric collection code grpc to this library. See #2323