Record aggregator metrics in event store

[jpraynaud]

Why

We need to store usage metrics in the event store of the aggregator in order to produce statistics reports. We want this feature to have a low impact on the performances of the aggregator.

What

Use the already existing Metrics service to create associated events (aggregated counter values) to be recorded in the existing event store at regular pace.

How

• Use the same migration system in the event store database than in other aggregator stores (in a different PR)
• Make the Metrics service export its counters in a HashMap<String, u32>
• Create a new UsageReporter service in the aggregator
  • Records delta of Metrics service counters between now and last call at regular pace
  • Produce events to be stored the event store when the delta of a counter is strictly greater than 0
  • Each event needs to embed the period used to compute the delta of the counter
  • Make the pace of reporting a configuration option of the aggregator
• Create a new runbook and SQL view for retrieval of:
  • Total counter values split by day (each row has date, counter_name, value)