pybind/mgr/prometheus: add Prometheus metric collector

pybind/mgr/prometheus: add Prometheus metric collector

This implements a Prometheus 'text' endpoint with cherrypy.
All paths return the metrics (as the Prometheus project's
Python client server does).

1) the newest stat is used for all counters/gauges
2) histograms are not handled
3) the timestamp assigned by Prometheus is the time of query, not
   the time collected by Ceph (long story), but this is now
   easily changed if necessary (by adding the timestamp in ms to
   the end of each reporting line, in Go float format).

Signed-off-by: Dan Mick <dan.mick@redhat.com>