Add mapping rules functionality

[neumayer]

We currently produce 1:1 rules from dropwizard metrics. This leads to things encoded in the metric name that should not be according to prometheus best practices (i.e. things should be labels rather than part of the metric name).

In this commit functionality is added to provide a mapping from metric names to labels, similar to the statsd_exporter approach:

https://github.com/prometheus/statsd_exporter

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[neumayer]

Things I know of for now:

• There's only a mapping for one of the existing metrics, I'd provide setup for all
• It's not configurable, I'd hide this behind a flag (that's not enabled as a standard)
• Also I know little about graylog internals (and metrics other than prometheus)

[jalogisch]

@neumayer would you please sign the CLA for all commits - after that is clean we are able to look into this.

Without the CLA sign for all commits I would close this PR in the next week.

[jalogisch]

@neumayer as you did not submitted the CLA for all changes we are not able to merge this PR.

Please re-submit it with a CLA included for all changes.

Thank you