mgr/prometheus: only turn - into _minus at end

mgr/prometheus: only turn - into _minus at end

...of perf counter name.

So that mds_mem_dir- becomes mds_mem_dir_minus, but
throttle-filestore_bytes becomes throttle_filestore_bytes.

At some point once this is all settled we should
probably just change Ceph's internal perf counter
naming to satisfy the major TSDB naming rules.

Signed-off-by: John Spray <john.spray@redhat.com>