prometheus: Fix metric resets

prometheus: Fix metric resets

This patch changes the way we reset metrics when collecting data in the
prometheus exporter module. With this patch, the exported metrics at any
point in time should align with a freshly started ceph-mgr module.

The patch also introduces locking mechanism to serialize the requests
and to avoid overwriting the metrics during multiple scrapes happenning
at the same time.

Signed-off-by: Boris Ranto <branto@redhat.com>