[confighttp] Add omitempty tag to fields

[evan-bradley]

Description

Depends on #12481.

Note that I didn't add the tag to any fields where we set a default value. This is because the zero value is significant in these cases and omitting it from the output would cause it to be set to the default instead of the zero value. We could fix this with a custom interface implemented by our config structs, but that will have to be a separate effort, and simply using the omitempty tag is still an improvement for most fields.

Link to tracking issue

Works toward #12191

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.16%. Comparing base (216046a) to head (96903c4).
Report is 28 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #12482   +/-   ##
=======================================
  Coverage   92.16%   92.16%           
=======================================
  Files         465      465           
  Lines       25201    25201           
=======================================
  Hits        23226    23226           
  Misses       1576     1576           
  Partials      399      399           

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