mgr/prometheus: Fix for MDS metrics

mgr/prometheus: Fix for MDS metrics

MDS metrics come in these forms:

mds_mem_dir #Directories
mds_mem_dir+ #Directories opened
mds_mem_dir- #Directories closed

In this case, continuing the trend of replacing all illegal characters with '_' results in…

mds_mem_dir #Directories
mds_mem_dir_ #Directories opened
mds_mem_dir_ #Directories closed

which is palpably a bad idea.

Suggested replacement for '+' = '_plus' seems fine, and a perusal of all metrics indicate that only MDS metrics end in '-' or '+' at this time.

Replacing '-' with '_minus' is probably less good for the general case, if anyone has a better idea…

I suppose another alternative would be to change MDS metrics so they don't use 'illegal' characters, but this also seems cumbersome and would break more third parties.

Fixes: http://tracker.ceph.com/issues/20899
Signed-off-by: Jeremy H Austin <jhaustin@gmail.com>