Precommit metadata checks

[kaidokert]

This check always runs for all modified files, and delegates
further filtering to the invoked script.
pre-commit exclude: filtering cannot be used here currently,
as it does not support multiple filters or overriding globals.

b/150775273

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6a39034) 58.54% compared to head (5831c37) 58.54%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2148   +/-   ##
=======================================
  Coverage   58.54%   58.54%           
=======================================
  Files        1911     1911           
  Lines       94585    94585           
=======================================
+ Hits        55371    55374    +3     
+ Misses      39214    39211    -3     

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[dahlstrom-g]

I was going to suggest using the commit 5831c37 title
“Add precommit check for METADATA files”
as the PR title which also becomes a commit title on submission.
https://cbea.ms/git-commit/#imperative