Conserve labels in query result, add selector support

[bobertrublik]

I've been using your mixin for quite some time but ran into an issue when I implemented label based alerting. The certificate alert queries currently strip away all labels except for the ones select in the by clause.

Then I had a closer look at the queries and in my opinion you don't need the aggregations for CertManagerCertExpirySoon and CertManagerCertNotReady. The certificates are already unique through the namespace + name combination and it's not possible to group them, therefore I removed the aggregations.

For CertManagerHittingRateLimits I looked at the metric and from what I see sum without (method, path) should be sufficient for aggregation.

Last I would find it very useful to set a selector variable similar how it's done in other mixins, examples:

• https://github.com/prometheus/node_exporter/blob/master/docs/node-mixin/config.libsonnet#L11
• https://github.com/prometheus-operator/prometheus-operator/blob/main/jsonnet/mixin/config.libsonnet#L3
• https://github.com/kubernetes-monitoring/kubernetes-mixin/blob/master/config.libsonnet#L20-L33

I refactored certManagerJobLabel to certManagerSelector and placed it in all alerts. I also updated the tests.

[bobertrublik]

However it might make sense to add sum without (pod, instance, container, endpoint) to all alerts to prevent unforseen behavior with pod restarts or cert-manager running in HA? The labels in the without clause could also be made configurable in case somebody runs into a regression.