Prom metrics

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PR Checklist

• Chart Version bumped
• CHANGELOG.md updated
• Variables and other changes are documented in the README.md

What this PR does / why we need it:
It is common for k8s services to export metrics in the prometheus format. This PR enables EventStore to do that by adding this prometheus exporter as a sidecar in the StatefulSet

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@ameier38 would appreciate a review, whenever you have some spare time

[ameier38]

Thanks for PR. This would be a great addition. Could you also add the new options to the README?

[ameier38]

This is great, just one more request. Would prefer to keep the image values consistent throughout the chart. See comments.

[zetaron]

Hi @ameier38,

I'd like to ping on this PR as it's been some time since your last review.
Would this be ok to merge?

I'm asking because our team is currently running Eventstore on kubernetes and would appreciate it, if monitoring was part of the package.