Add metrics for in-memory cache

[codecov-commenter]

Codecov Report

Merging #1014 into 5.x will increase coverage by 0.1%. The diff coverage is 91.9%.

@@           Coverage Diff           @@
##             5.x   #1014     +/-   ##
=======================================
+ Coverage   94.7%   94.7%   +0.1%     
=======================================
  Files        290     290             
  Lines      26792   26846     +54     
=======================================
+ Hits       25365   25420     +55     
+ Misses      1427    1426      -1     

Impacted Files	Coverage Δ
guillotina/contrib/cache/memcache.py	84.3% <72.8%> (-15.7%)	:arrow_down:
guillotina/contrib/cache/utility.py	83.0% <85.8%> (+3.5%)	:arrow_up:
guillotina/tests/test_metrics.py	100.0% <100.0%> (ø)

[vangheem]

coverage reporting isn't making any sense right here...