Remove restriction on setting both metadata and metadata_url

[dantecatalfamo]

Checklist for submitter

If some of the following don't apply, delete the relevant line.

• Changes file added for user-visible changes in changes/, orbit/changes/ or ee/fleetd-chrome/changes.
  See Changes files for more information.
• Added/updated automated tests
• A detailed QA plan exists on the associated ticket (if it isn't there, work with the product group's QA engineer to add it)
• Manual QA for all new/changed functionality

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 63.88%. Comparing base (1e3d735) to head (ffed1bf).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #26279      +/-   ##
==========================================
- Coverage   63.88%   63.88%   -0.01%     
==========================================
  Files        1632     1632              
  Lines      157962   157959       -3     
  Branches     4038     4038              
==========================================
- Hits       100908   100906       -2     
+ Misses      49154    49153       -1     
  Partials     7900     7900              

Flag	Coverage Δ
backend	64.72% <ø> (-0.01%)	⬇️

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