[18.03] Add metrics for store object types

[thaJeztah]

backport of #2673 for 18.03

(this was already backported to 17.06 through #2679)

git checkout -b 18.03-backport-add-more-metrics upstream/bump_v18.03 
git cherry-pick -s -S -x b67abf85fcae39716863aec108144d9408ee9fbe

cherry-pick was clean; no conflicts

Adds metrics for most of the user-facing store object types. Allows the
user to keep track of how many objects are in the object store at any
given time.

(cherry picked from commit b67abf8)

[codecov]

Codecov Report

Merging #2703 into bump_v18.03 will decrease coverage by 0.3%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           bump_v18.03    #2703      +/-   ##
===============================================
- Coverage        61.82%   61.52%   -0.31%     
===============================================
  Files              134      134              
  Lines            21820    21820              
===============================================
- Hits             13491    13425      -66     
- Misses            6888     6951      +63     
- Partials          1441     1444       +3

[cyli]

LGTM