Clarify meaning of metrics on monitoring.md

[cep21]

Hi,

I'm reading https://github.com/fluxcd/helm-operator/blob/master/docs/references/monitoring.md and the difference between release_phase_duration_seconds and release_duration_seconds isn't very clear to me. The only difference seems to be "Release" and "Release Phase", which I'm not sure where they are defined. Is it possible to add some clarification comments around how the two metrics are different?

Thanks!

[hiddeco]

A release consists of one or multiple phases.

The *_release is the total duration of the whole releases process, this includes all preparations and the one or many phases it goes through.

The _*release_phase is the duration of a single phase of type x for that release.