feat(insights): Adds metrics to web vitals issue detection task

[edwardgou-sentry]

Adds metrics to the web vitals issue detection task to record:

• number of projects checked
• number of sub tasks dispatched for each project
• number of issues created by web vital issue type
• number of issues rejected by reason

[codecov]

Codecov Report

❌ Patch coverage is 96.15385% with 1 line in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
src/sentry/tasks/web_vitals_issue_detection.py	95.65%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master   #103538      +/-   ##
===========================================
+ Coverage   80.57%    80.62%   +0.04%     
===========================================
  Files        9255      9250       -5     
  Lines      395176    394960     -216     
  Branches    25198     25161      -37     
===========================================
+ Hits       318407    318427      +20     
+ Misses      76340     76104     -236     
  Partials      429       429