Add logs to track price updates

[mjh1]

To help us try to understand the price mismatch issues we've been seeing.

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 31.65300%. Comparing base (9f123db) to head (43c9835).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
eth/watchers/pricefeedwatcher.go	50.00000%	1 Missing ⚠️

Additional details and impacted files

@@                 Coverage Diff                 @@
##              master       #3826         +/-   ##
===================================================
- Coverage   31.67769%   31.65300%   -0.02469%     
===================================================
  Files            159         159                 
  Lines          39018       39020          +2     
===================================================
- Hits           12360       12351          -9     
- Misses         25763       25776         +13     
+ Partials         895         893          -2     

Files with missing lines	Coverage Δ
eth/watchers/pricefeedwatcher.go	66.98113% <50.00000%> (-0.32656%)	⬇️

... and 5 files with indirect coverage changes

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Files with missing lines	Coverage Δ
eth/watchers/pricefeedwatcher.go	66.98113% <50.00000%> (-0.32656%)	⬇️

... and 5 files with indirect coverage changes

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[j0sh]

LGTM, but I think things can't be deployed until SRE figures out what broke the ffmpeg build in Docker: #3825