chore: add stats logging to thumbnail api

[mistercrunch]

SUMMARY

Adding a few events to related to thumbnail generation. The main driver here is to be able to compute a thumbnail_cache_hit_rate

• DashboardRestApi.thumbnail.async: endpoint is hit, 202 is returned and async process is triggered
• DashboardRestApi.thumbnail.redirect: thumbnail exists for dashboard, redirecting to the right hash (digest)
• DashboardRestApi.thumbnail.from_cache: indicates the thumbnail was loaded from cache (following the redirect)

All equivalent events for ChartRestApi are also logged the same way.

Logically cache_hit_rate = from_cache/(async+from_cache)

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TESTING INSTRUCTIONS

Tested thoroughly on docker-compose locally, made sure all the events logged through the dummy stats logger looked as expected

[codecov]

Codecov Report

Merging #16133 (9a16605) into master (85ae8e3) will decrease coverage by 0.20%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #16133      +/-   ##
==========================================
- Coverage   76.84%   76.63%   -0.21%     
==========================================
  Files         995      995              
  Lines       52884    52890       +6     
  Branches     6721     6721              
==========================================
- Hits        40638    40533     -105     
- Misses      12021    12133     +112     
+ Partials      225      224       -1     

Flag	Coverage Δ
hive	?
javascript	71.23% <ø> (ø)
mysql	81.58% <100.00%> (+0.04%)	⬆️
postgres	81.61% <100.00%> (+<0.01%)	⬆️
presto	?
python	81.69% <100.00%> (-0.43%)	⬇️
sqlite	81.25% <100.00%> (+0.04%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
superset/charts/api.py	85.82% <100.00%> (+0.11%)	⬆️
superset/dashboards/api.py	93.03% <100.00%> (+0.07%)	⬆️
superset/db_engines/hive.py	0.00% <0.00%> (-82.15%)	⬇️
superset/db_engine_specs/hive.py	69.80% <0.00%> (-16.87%)	⬇️
superset/db_engine_specs/presto.py	83.47% <0.00%> (-6.91%)	⬇️
superset/views/database/mixins.py	81.03% <0.00%> (-1.73%)	⬇️
superset/connectors/sqla/models.py	88.08% <0.00%> (-1.67%)	⬇️
superset/db_engine_specs/base.py	87.98% <0.00%> (-0.39%)	⬇️
superset/models/core.py	89.61% <0.00%> (-0.26%)	⬇️
superset/utils/core.py	88.09% <0.00%> (-0.13%)	⬇️
... and 1 more

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[craig-rueda]

LGTM

[rosemarie-chiu]

🏷 2021.31