Hook tool for sending metrics from charm.
Merge pull request #640 from tasdomas/metrics-in-hook-context

Hook tool for sending metrics from charm.

A hook tool metric-add is made available to charm authors for sending metrics to state.
implemented the sender worker for metrics
added test and comments after first round of reviews
second round of of review, support bulk calls in setting sent,
added more mocking in tests
support bulk calls in the sendmetrics api call