Remove Alertmanager API v1

[simonpasquier]

Alertmanager API v2 is available since v0.16.0 and it's probably fair to say that it's (at least) feature-equivalent to the API v1 since v0.19.0. At the time when v0.16.0 was released, the CHANGELOG stated that v1 would be removed in v0.18.0. Obviously it hasn't happened yet and this issue is to discuss if/when it should happen.

cc @roidelapluie @beorn7

xref prometheus/prometheus#8398

[beorn7]

Good to know that the API v1 removal had been announced long ago.

Yes, it would be great to retire v1 as soon as reasonably possible. It's existence caused us a lot of headache when adding new features.

[beorn7]

Also, let's remember to remove any json tags and json marshalling code that are not needed anymore after the removal (if that applies to any).

[gotjosh]

I'll be happy to take care of this.

[beorn7]

Great. Just note that we should coordinate this with prometheus/prometheus#8398 , i.e. ideally we only remove the v1 API once Prometheus defaults to the v2 API to avoid friction.

[gotjosh]

Consensus was reached during the Alertmanger Working Group that we'll proceed with this but keep the api/v1/alerts endpoint as a redirect to the api/v2/alerts endpoint.