feat: latest snapshot timestamp metric

[caarlos0]

added a new metric that reports the timestamp of the latest SUCCESS/PARTIAL snapshot.

This should make it easier to alert things like X time since the last snapshot, even more helpful when you have some sort of repository rotation (e.g. each month has a new repository / s3 bucket, and we keep up to 2 old repositories there).

with this metric the alert rule could be:

(time() - max without (repository) elasticsearch_snapshot_stats_latest_snapshot_timestamp) > X

Thanks!

[zwopir]

as commented (metric name). Looks good otherwise

[caarlos0]

done.

[zwopir]

I'd like to have the information that this is a timestamp in the metric name. It's a bit longer, but just _seconds doesn't provide the information. I'm also fine with _ts_seconds.

[caarlos0]

@zwopir done

[grobie]

Great, thanks @caarlos0! I'm happy to see the explicit names, with tab completion in prometheus and grafana, abbreviations should be rather avoided.

[caarlos0]

Me too 🙏

[zwopir]

thanks again for the PR, @caarlos0 !