Template out the grafana dashboard so we can change the cluster prometheus label

[onedr0p]

As noted in cloudnative-pg/cloudnative-pg#2501 we should be able to change the default cluster label to work with Prometheus Federation.

Also as I mentioned in cloudnative-pg/cloudnative-pg#2501 (comment) and cloudnative-pg/cloudnative-pg#3075 (comment) I can't even get the relabellings to work, so there's also a problem or something is wrong with my config, nobody has responded 😢

[danielloader]

This is also true for a grafana cloud install using alloy since it also reserves the cluster label to refer to kubernetes clusters.

[itay-grudev]

I'm waiting for a solution of cloudnative-pg/cloudnative-pg/issues/2501 after which we'll patch the dashboard.