[Feature] List down all the metrics necessary for maintaining multinode

[shreyas-s-rao]

Feature (What you would like to be added):
Create a collective list of all the metrics that is needed to maintain multi-node ETCD. Update the file with the list. This is a running document that will capture all the metrics that will be exposed through prometheus for mutinode ETCD.

Motivation (Why is this needed?):
Once etcd-druid starts managing multi-node etcd clusters, it would perform operations such as cluster scale-up, scale-down, recovery quorum losses, forced restorations, etc. Currently druid does not expose any metrics about its operations, and these metrics will become imperative for the multi-node story, especially for understanding and debugging druid behaviour during etcd cluster failures.

Approach/Hint to the implement solution (optional):