[Explore] including auto generated avg metrics in druid

[gabe-lyons]

AVG is a more in-depth post aggregation in druid, and I didn't include that in the v0 of adhoc metrics (#4663). However, that PR did exclude auto-generated avg metrics in druid, meaning there's no way to access and avg-based metric in explore currently. This PR whitelists auto generated avg metrics specifically for druid so those calculations can still be accessed.

test plan:

• view a sqla datasource with auto generated avg metrics. verify they do not appear
• view a druid datasource with autogenerated avg metrics. verify they do appear
• run tests

reviewers:
@michellethomas @mistercrunch @williaster @graceguo-supercat

[codecov-io]

Codecov Report

Merging #4718 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #4718      +/-   ##
==========================================
+ Coverage   71.87%   71.88%   +<.01%     
==========================================
  Files         204      204              
  Lines       15323    15326       +3     
  Branches     1177     1176       -1     
==========================================
+ Hits        11014    11017       +3     
  Misses       4306     4306              
  Partials        3        3

Impacted Files	Coverage Δ
...pts/explore/components/controls/MetricsControl.jsx	78.91% <100%> (+0.43%)	⬆️

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[michellethomas]

lgtm