Pure metrics

[zhilong-xu]

What this PR does / why we need it:

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #

Special notes for your reviewer:

Release note:

[codecov]

Codecov Report

Merging #911 (3957d42) into master (bfd1318) will increase coverage by 0.11%.
The diff coverage is 88.97%.

@@            Coverage Diff             @@
##           master     #911      +/-   ##
==========================================
+ Coverage   71.35%   71.47%   +0.11%     
==========================================
  Files         182      182              
  Lines       21400    21532     +132     
  Branches     3271     3291      +20     
==========================================
+ Hits        15269    15389     +120     
- Misses       5097     5103       +6     
- Partials     1034     1040       +6     

Impacted Files	Coverage Δ
delfin/drivers/pure/flasharray/pure_flasharray.py	86.20% <88.37%> (+0.66%)	⬆️
delfin/drivers/pure/flasharray/consts.py	100.00% <100.00%> (ø)
delfin/drivers/pure/flasharray/rest_handler.py	40.24% <100.00%> (+1.49%)	⬆️
delfin/drivers/fake_storage/__init__.py	94.41% <0.00%> (+0.76%)	⬆️

[wisererik]

LGTM

[joseph-v]

LGTM