feat(broker): Add a health check endpoint based on upkeep interval

[evanh]

Add a health check endpoint to the tonic server, and have the health go down if the upkeep thread is
taking too long to run. Track metrics about being healthy vs. unhealthy.

[codecov]

Codecov Report

❌ Patch coverage is 34.09091% with 29 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.17%. Comparing base (1a176aa) to head (14dc985).
⚠️ Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
src/upkeep.rs	0.00%	22 Missing ⚠️
src/main.rs	0.00%	7 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #469      +/-   ##
==========================================
- Coverage   88.50%   88.17%   -0.34%     
==========================================
  Files          20       20              
  Lines        5335     5293      -42     
==========================================
- Hits         4722     4667      -55     
- Misses        613      626      +13     

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