Configure alert_relabel_configs for prometheus alertmanagers

Summary

Let user configure alert_relabel_configs for prometheus alertmanagers

Proposal

I have pretty complicated prometheus setup and has to use such configuration for

alerting:
  alert_relabel_configs:
    - target_label: monitor
      replacement: gitlab
    - target_label: environment
      replacement: "prod"
  alertmanagers:
    - scheme: https
      basic_auth:
        username: "XXX"
        password: "XXX"
      static_configs:
      - targets:
        - alertmanager.company.name

I can configure second part of alerting section but can't do it for first part. So i will not have labels describing the source and meta info about where i got it from

References

!2803 (merged) was the first step i can manage to add any info from internal prometheus to my alert infra