grpc integration

[bonnefoa]

What does this PR do?

Create a gRPC integration by pulling metrics from channelz.

Motivation

Channelz is the only way to get insight on the channel/subchannel state of gRPC client and servers. We can extract interesting metrics from channelz.

Additional Notes

Review checklist (to be filled by reviewers)

• [ ] Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• [ ] PR title must be written as a CHANGELOG entry (see why)
• [ ] Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• [ ] PR must have changelog/ and integration/ labels attached

[github-actions[bot]]

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