feat(subscriptions): Update command args to accept metrics entities

[ahmedetefy]

Adds to the subscriptions_executor and subscriptions_scheduler
commands the supported metrics entity choices

[codecov-commenter]

Codecov Report

Merging #2302 (0c056a1) into master (6f92898) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #2302   +/-   ##
=======================================
  Coverage   92.97%   92.97%           
=======================================
  Files         563      563           
  Lines       25932    25932           
=======================================
  Hits        24109    24109           
  Misses       1823     1823           

Impacted Files	Coverage Δ
snuba/cli/subscriptions_executor.py	53.48% <ø> (ø)
snuba/cli/subscriptions_scheduler.py	57.77% <ø> (ø)

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[lynnagara]

@ahmedetefy This is not really ready for use yet, do you need these for a particular reason? Or just making sure these options are eventually available?

[ahmedetefy]

@lynnagara
Yep, just making sure these options are available eventually.