Allow restriction of running instance to specific cluster or clusters

[jstewart612]

Is your feature request related to a problem? Please describe

The all-or-none approach of this exporter is a SPOF. I tend to like to avoid those in my environment.

Describe the solution you'd like

As such, I would like the ability to lock down a running copy of the exporter to only gather information on specific cluster or clusters.

Describe alternatives you've considered

I have worked around this problem with metricRelabelings in the helm chart. I would like to stop having to do this and putting splat operators in my IAM policies.

Additional context

None required.

[vmercierfr]

Thanks for reporting your usage, I agree that is required feature.

If you don't mind, I'll close this issue in favor of #94, which is similar.