Adds custom labels for every cluster

[anbarasantr]

This fixes 34. This would help to specify custom / additional labels which could be specified per cluster.

Example:

kafka-lag-exporter {
  port = 9999
  lookup-table-size = 60
  clusters = [
    {
      name = "us-east_ny_a-cluster"                                   
      bootstrap-brokers = "us-east_ny_a-1.cluster-a.xyzcorp.com:9092,us-east_ny_a-2.cluster-a.xyzcorp.com:9092,us-east_ny_a-3.cluster-a.xyzcorp.com:9092"
      labels {
        location = "ny"
        zone = "us-east"
      }
    }
  ]
}

This would simplify the Prometheus job config. Because now Prometheus relabelling tricks must be applied to get this labels.

[lightbend-cla-validator]

Hi @anbarasantr,

Thank you for your contribution! We really value the time you've taken to put this together.

Before we proceed with reviewing this pull request, please sign the Lightbend Contributors License Agreement:

http://www.lightbend.com/contribute/cla

[anbarasantr]

@seglo I have signed the agreement, can you please review this PR ?

[seglo]

Thanks for the PR @anbarasantr. This looks good! I've made some mostly minor requests.

[anbarasantr]

@seglo Thanks for the suggestions. I have fixed it.

[seglo]

LGTM. Thanks for the contribution.