Split monolith static monitor target file into per-cluster conf

[Vonng]

Currently. If you are using static prometheus target discovery.

The target file is a monolith lies in /etc/prometheus/targets/all.yml.

It's good to split that monolith file into small files, for example, one yml file per cluster.

Besides, for people who are using per-cluster style inventory, this could work, too.

The targets files would be:

/etc/prometheus/targets/pg-test.yml
/etc/prometheus/targets/pg-meta.yml
/etc/prometheus/targets/pg-other-cluster.yml