Feat/telemetry grouped counter

[codeflyer]

Add a metric of type GROUPED_COUNT

      telemetry.createMetric('counter', 'COUNTER', METRIC_GROUPED_COUNT)

That can be updated using:

telemetry.increaseCountWithKey('counter', '{id="123"}')
telemetry.decreaseCountWithKey('counter', '{id="123"}')

end exports:

     # HELP counter_grouped_count_total COUNTER (grouped-count)
      # TYPE counter_grouped_count_total counter
      counter_grouped_count_total{id="123"} 1 now
      counter_grouped_count_total{id="456"} 2 now

[simone-sanfratello]

I'd add documentation for the new part