Adding Prometheus /metrics endpoint and some data
    
    Adding the Prometheus endpoint /metrics so that when one day we will
    have a Prometheus instance running for our services the metrics will
    already be available.
    Added:
    * basic metrics automatically provided by Prometheus
    * counter for exceptions in the waiverdb consumer (exception receiving
      a new waiver)
    * counter for exceptions in the resultsdb consumer (exception receiving
      a new result)
    * counter for exceptions in the decision API
    * latency for the decision API