Feature: Instrument Alertmanager for distributed tracing

[hairyhenderson]

It'd be ideal to have Alertmanager instrumented for tracing (with OpenTelemetry), to help when operating it at scale. Prometheus is already instrumented, and so it would make sense to instrument Alertmanager in a similar way.

Once Alertmanager is instrumented, I would also like to add trace propagation over SMTP for e-mail notifications. See this PR in grafana/grafana for a related approach.

/cc @gotjosh

[gotjosh]

This looks good to me!

[hairyhenderson]

Alright! I'll get started on a PR for this.

[eyazici90]

Hey @hairyhenderson, I see the PR is there already being review. We would benefit a lot from this feature. Is there anything I can do to help to push this further? :D 🙏