Add performance counter hooks

[lucasdicioccio]

Operators would be super happy to debug services doing thousands queries per second with some ekg or prometheus counters and latency metrics.

What we should add:

• hooks supporting user-defined IO-actions to count:
  • start of a query
  • end of a query (+duration & gRPC service/method name)
• then it is somewhat trivial to add packages for ekg and for prometheus to provide such IO actions with their own implementations with the following metrics
  • number of processed queries and load
  • number of inflight-queries
  • statistics about durations (average, max)