promdump: k8s metrics grab

[eugeneckim]

From a discussion related to grabbing k8s metrics, we have a number of customers (who open tickets a lot) that run promdump against k8s setups. There are a certain set of metrics which are available that don't have exported_instance in them, but can be useful to have.

https://yugabyte.slack.com/archives/C01HFHVV3NV/p1712803772802329?thread_ts=1712791089.498809&cid=C01HFHVV3NV

This talks a bit about what unique metrics there are. In most cases people key off of namespace but perhaps there is some better label. It'd be nice to check if these are available and grab if there without a user needing to know if they are/aren't on a k8s setup.