[alertmanager] Add optional seperate ingress per replica

[knutgoetz]

What this PR does / why we need it

This pull request adds the ability to add a separate ingress for each Alertmanager replica. This allows for the replicas to be exposed and used as a highly available cluster by Prometheus instances outside the cluster.

Special notes for your reviewer

This feature was already implemented in the kube-prometheus-stack chart and has been adapted for this implementation. There are a few key differences to note:

• This implementation does not support Ingress apiVersion < networking.k8s.io/v1
• You cannot set a static nodePort for the services per replica. This is because statically assigning the port can lead to collisions when using more than one replica. If you are using only one replica, you can use the default service.

Checklist

• DCO signed
• Chart Version bumped
• Title of the PR starts with chart name (e.g. [prometheus-couchdb-exporter])

[monotek]

Can't you use a new k8s service which uses one of the pods?
So only one ingress would be needed.

[knutgoetz]

Not sure, if i understand your suggestion correctly.
I need to send an alert from prometheus to every alertmanager pod to achieve high availability for the alertmanager cluster. It does not suffice to send the alert only to of the replicas, i.e. use simply one ingress+service which does the load balancing. That's why an extra service per replica is needed.

I could reach each of these services via one ingress with stuff like path based routing (different path per service, then deleting the path prefix) or with an ingress controller which supports mirroring the request to multiple services but this would need some additional work and relies on the specific ingress controller.
I thought that having one ingress per service is the simplest way to go.

[knutgoetz]

Not sure if my previous comment clarified anything @monotek :)
I know this not the most urgent feature the community is waiting for - especially compared to the amount of code that is added. If you think this does not belong in the community chart, we will add the services/ingresses in our own setup and close this PR which is fine as well.