Expose alarms as metrics

[stek29]

Currently the only way to know about alarms is to issue an maitenance/alarm grpc call to list currently active alarms (by using etcdctl alarm list, for example).

It would be great to have expose etcd alarms in prometheus metrics so they're easier to monitor and know about.

[yuzhiquan]

/assign @yuzhiquan

[stek29]

@ahrtr @serathius is there any chance to get this in next 3.5.x release?
I'm not familiar with etcd release process and backporting, and I haven't been able to find any docs on it.

[ahrtr]

@stek29 FYI. https://github.com/etcd-io/etcd/blob/main/Documentation/contributor-guide/release.md#patch-version-release

Usually we only cherry pick bug fixes and security patches. So normally we shouldn't backport the PR #14276 to 3.5.

[stek29]

@ahrtr got it, we'll stick to custom exporter until 3.6 is mature enough for us to use in production then