New metric for empty coverage

[nharper285]

Summary of the Pull Request

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PR Checklist

• Applies to work item: #xxx
• CLA signed. If not, go over here and sign the CLI.
• Tests added/passed
• Requires documentation to be updated
• I've discussed this with core contributors already. If not checked, I'm ready to accept this work might be rejected in favor of a different grand plan. Issue number where discussion took place: #xxx

Info on Pull Request

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Validation Steps Performed

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[codecov-commenter]

Codecov Report

Merging #3610 (b84b3b5) into main (d50fd48) will increase coverage by 0.00%.
Report is 1 commits behind head on main.
The diff coverage is 83.33%.

@@           Coverage Diff           @@
##             main    #3610   +/-   ##
=======================================
  Coverage   39.95%   39.96%           
=======================================
  Files         303      303           
  Lines       37724    37729    +5     
  Branches     1738     1738           
=======================================
+ Hits        15073    15077    +4     
- Misses      22330    22331    +1     
  Partials      321      321           

Files	Coverage Δ
...c/agent/onefuzz-task/src/tasks/coverage/generic.rs	88.71% <100.00%> (+0.65%)	⬆️
src/agent/onefuzz-telemetry/src/lib.rs	17.27% <0.00%> (-0.06%)	⬇️

... and 1 file with indirect coverage changes