pybind/mgr/prometheus: add Metrics class to manage Metric instances

pybind/mgr/prometheus: add Metrics class to manage Metric instances

The central change of this commit is that per-daemon metrics are now
managed by first appending the metric (using Metrics.append) to a
staging area. Then the metrics for specific paths (metric names) are
overwritten by the staged metrics (by calling Metrics.reset). This gets
rid of metrics from daemon that are no longer in the cluster. I.e. when
ceph no longer reports metrics for one OSD daemon (because it was
removed from the cluster) the prometheus module will no longer export
metrics for that daemon.

Signed-off-by: Jan Fajerski <jfajerski@suse.com>