Scheduler listener/runner: add statsd probes

Add statsd probes for the amount of

• tasks scheduled per task type by the runner
• events processed per ??? by the listener

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Migrated from T2133 (view on Phabricator)

mentioned in merge request !119 (closed)

added Metrics/monitoring Scheduling utilities Sprint 2019/12 (Monitor and Conquer) labels

added priority:High label

added priority:Normal label and removed priority:High label

Closed by !119 (closed)

assigned to @douardda

closed