promgrpc merge - change 1

[piotrkowalczuk]

This PR is related to #60, it introduces:

• No global state (client metrics).
• Shared monitor struct.
• Changed metric names to be more consistent and intuitive. breaking
  • counters
    • started_total -> requests_total
    • handled_total -> responses_total
    • msg_received_total -> received_messages_total
    • msg_sent_total -> sent_messages_total
  • gauges (new)
    • in_flight_requests // partially
    • connections // partially
  • histograms
    • handling_seconds->request_duration_histogram_seconds
• Request duration metric (histogram) is not optional anymore. breaking
• clientReporter API become private
• CounterOption and HistogramOption replaced by CollectorOption breaking
  • WithConstLabels
  • WithNamespace
  • WithSubsystem
  • WithBuckets

TODO:
[ ] if accepted, apply similar changes to ServerMetrics