Make ETag a required property of the URL Metric

[ShyamGadde]

Summary

Fixes #1819

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.96%. Comparing base (0615c18) to head (bd642b4).
Report is 20 commits behind head on trunk.

Additional details and impacted files

@@            Coverage Diff             @@
##            trunk    #1824      +/-   ##
==========================================
- Coverage   65.96%   65.96%   -0.01%     
==========================================
  Files          88       88              
  Lines        6876     6872       -4     
==========================================
- Hits         4536     4533       -3     
+ Misses       2340     2339       -1     

Flag	Coverage Δ
multisite	65.96% <100.00%> (-0.01%)	⬇️
single	38.66% <33.33%> (+0.02%)	⬆️

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[github-actions]

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Co-authored-by: ShyamGadde <shyamgadde@git.wordpress.org>
Co-authored-by: westonruter <westonruter@git.wordpress.org>

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[felixarntz]

@ShyamGadde Thanks, LGTM!

[westonruter]

Thank you!