Make promscrape.cluster.membersCount discoverable

[AndrewChubatiuk]

Is your feature request related to a problem? Please describe

VMAgent sharding requires setting -promscrape.cluster.membersCount flag, which should be updated every time deployment/statefulset scales up or down, which requires all pods to be recreated. Being able to discover this value dynamically would help to cope with this issue

Related issues VictoriaMetrics/operator#924, VictoriaMetrics/operator#1101

Describe the solution you'd like

Add interface and some implementations of this interface for K8s, Consul, etc with flags or configuration file, that helps to provide additional settings for each of discovery mechanisms which can be enabled instead of setting a static -promscrape.cluster.membersCount value

Describe alternatives you've considered

No response

Additional information

No response