Update documentation on statsd metrics generated.
Update documentation on statsd metrics generated.

Updates the metrics documentation to reflect the statistics generated on
the scheduler code. Add the hierarchy of metrics, type of each metric
and unit of measure.

Co-Authored-By: Danilo Ramalho <ce5b42a5beb17939794f3fabc6da3eb270367200@thoughtworks.com>
Change-Id: I9eb98efe85317fba404d9c5c087c33762f7eb4ab

gooddata/zuul,gooddata/zuul,wikimedia/integration-zuul,wikimedia/integration-zuul,gooddata/zuul,wikimedia/integration-zuul,gooddata/zuul,wikimedia/integration-zuul,gooddata/zuul