[collector] Allow adding extra labels to Prometheus alerts

[wrdls]

In our company we need to add something like a team or domain label to our alerts to route them to the correct owner.

https://github.com/open-telemetry/opentelemetry-helm-charts/blob/main/charts/opentelemetry-collector/templates/prometheusrule.yaml#L23-L24

E.g. the above would need to become something like this for us.

      labels:
        severity: critical
        team: sre

Helm templating wise this would be very similar as this https://github.com/open-telemetry/opentelemetry-helm-charts/blob/main/charts/opentelemetry-collector/templates/prometheusrule.yaml#L9-L11

I'm willing to contribute if you think this would be a good thing to have.

[TylerHelmuth]

Adding something to prometheusRule.defaultRules to add labels to every default rule seem ok, but if you need specific labels for specific rules you should use prometheusRule.groups.