vmagent: expose each shard of the cluster under different address/service name

[hagen1778]

When in cluster mode, vmagent splits the list of targets among the configured shards. One shard starts to scrape only 1/N fraction of all configured targets, and so it shows on /targets page as well.

Each vmagent shard has its own share of targets to scrape. But there is no way to view to the global list of all configured targets via UI.

The issue can be solved via https://github.com/VictoriaMetrics/VictoriaMetrics/issues/4018

As workaround, each shard of vmagent cluster can be exposed under different service name, so user can have access to all vmagent shards and check their UIs manually.

[f41gh7]

This work-around is out of the goal of operator configuration. vmagent exposed via external Ingress rule not managed by operator.

I'll add an examples later, but it's better to wait for actual implementation at vmagent ui for https://github.com/VictoriaMetrics/VictoriaMetrics/issues/4018.