Notify: Several improvements around the suppressed_total metric and…

[gotjosh]

… logs for muting.

• Fix the description of the metric to include the fact that the metric is also incremented when the alert is silenced or inhibited.
• Make the reason constants public so that they can be-reused in projects that consume the Alertmanager as a library
• Add the reason to the debug line for muting.

[grobinson-grafana]

LGTM!

[grobinson-grafana]

I also tested the logging for silences and inhibition rules and it looks good:

inhibit_rules:
  - source_matchers:
    - foo=bar
    target_matchers:
    - bar=baz

ts=2024-02-15T11:08:03.335Z caller=dispatch.go:516 level=debug component=dispatcher aggrGroup={}:{} msg=flushing alerts="[[3fff2c2][active] [6ad8c19][active]]"
ts=2024-02-15T11:08:03.335Z caller=notify.go:561 level=debug component=dispatcher msg="Notifications will not be sent for muted alerts" alerts=[[6ad8c19][active]] reason=inhibition
ts=2024-02-15T11:08:03.335Z caller=silence.go:174 level=debug msg="determined current silences state" now=2024-02-15T11:08:03.335969Z total=1 active=1 pending=0
ts=2024-02-15T11:08:03.336Z caller=notify.go:561 level=debug component=dispatcher msg="Notifications will not be sent for muted alerts" alerts=[[3fff2c2][active]] reason=silence