Add CountDistinctOkInstanceKeys to discovery aggregate metrics.
Add CountDistinctOkInstanceKeys to discovery aggregate metrics.

We had a count of the different number of hosts seen in the period,
and the different number of failures but given that in the monitoring
period successful and failed discoveries may happen for the same
host we can't infer the missing metric. This patch adjusts that by
providing the 3 values:

* CountDistinctInstanceKeys
* CountDistinctOkInstanceKeys (new)
* CountDistinctFailedInstanceKeys

enisoc/orchestrator,github/orchestrator,github/orchestrator,github/orchestrator,github/orchestrator,enisoc/orchestrator,enisoc/orchestrator,enisoc/orchestrator