mgr/prometheus: Fix pg_* counts

mgr/prometheus: Fix pg_* counts

Currently, the pg_* counts are not computed properly. We split the
current state by '+' sign but do not add the pg count to the already
found pg count. Instead, we overwrite any existing pg count with the new
count. This patch fixes it by adding all the pg counts together for all
the states.

It also introduces a new pg_total metric for pg_total that shows the
total count of PGs.

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