Plan to GA etcd gprc-proxy --experimental-enable-grpc-logging

[nwnt]

What would you like to be added?

This issue proposes that we eventually make --experimental-enable-gprc-logging. It's also for tracking actions required to GA.

The first step is to move it to feature gates, but documentation will have to be added for it as well.

Why is this needed?

The feature is experimental (alpha?). This proposes that we make it GA.

[ahrtr]

This is just a flag for a pretty simple feature to enable the debug log for grpc-proxy, we already have test cases (thx @nwnt for adding the test), also manually verified.

I think we just need to follow the similar pattern as #19282 to migrate the experimental flag to a flag without the experimental- prefix, and completely remove the experimental flag in 3.8 (or 4.0 if we don't have 3.8).