Improve delete all for the memcache driver

[pfreixes]

• Do not execute internally the delete all when len of the keys is 0
• Observe, if metrics available, the size of the list of keys

[codecov-io]

Codecov Report

Merging #1043 (431c40a) into 5.x (3e96a3d) will decrease coverage by 0.7%. The diff coverage is 100.0%.

@@           Coverage Diff           @@
##             5.x   #1043     +/-   ##
=======================================
- Coverage   94.7%   94.1%   -0.6%     
=======================================
  Files        296     296             
  Lines      27218   26827    -391     
=======================================
- Hits       25770   25237    -533     
- Misses      1448    1590    +142     

Impacted Files	Coverage Δ
guillotina/contrib/memcached/driver.py	86.9% <100.0%> (+1.9%)	:arrow_up:
...uillotina/tests/memcached/test_memcached_driver.py	100.0% <100.0%> (ø)
guillotina/tests/memcached/test_metrics.py	100.0% <100.0%> (ø)
guillotina/tests/test_cockroach.py	17.7% <0.0%> (-77.9%)	:arrow_down:
guillotina/db/storages/cockroach.py	27.9% <0.0%> (-48.1%)	:arrow_down:
guillotina/db/strategies/dbresolve.py	68.5% <0.0%> (-31.5%)	:arrow_down:
guillotina/db/factory.py	85.3% <0.0%> (-7.7%)	:arrow_down:
guillotina/contrib/cache/api.py	54.6% <0.0%> (-6.9%)	:arrow_down:
guillotina/json/serialize_value.py	82.2% <0.0%> (-3.5%)	:arrow_down:
guillotina/contrib/pubsub/utility.py	84.5% <0.0%> (-2.3%)	:arrow_down:
... and 93 more

[vangheem]

Also, just a real simple flake issue:

guillotina/tests/memcached/test_memcached_driver.py:137:100: E231 missing whitespace after ','

[pfreixes]

@vangheem Feedback applied, PTAL

[vangheem]

5.3.61 released.