List used labels by metric in grafana

[IuryAlves]

This PR adds the labels used my each metric when running the cortex-tools analyse grafana command.

It adds a new field to the output called labels_by_metric that has the following format:

      "labels_by_metric": {
        "<metric_name>": [
          "label_1",
          "label_2",
            ...
        ]
      }   

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