testutil/promrated: add metric to count errors from rated api

[LukeHackett12]

Rated recently added authentication and this has been failing silently as the error was just being logged

category: bug
ticket: #1738

[codecov]

Codecov Report

Base: 54.69% // Head: 54.81% // Increases project coverage by +0.12% 🎉

Coverage data is based on head (5fca451) compared to base (a3e5bdf).
Patch coverage: 28.57% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1739      +/-   ##
==========================================
+ Coverage   54.69%   54.81%   +0.12%     
==========================================
  Files         158      162       +4     
  Lines       20484    20825     +341     
==========================================
+ Hits        11204    11416     +212     
- Misses       7794     7885      +91     
- Partials     1486     1524      +38     

Impacted Files	Coverage Δ
testutil/promrated/promrated.go	0.00% <0.00%> (ø)
testutil/promrated/rated.go	54.66% <33.33%> (-2.48%)	⬇️
cmd/enr.go	8.16% <0.00%> (-6.94%)	⬇️
cluster/ssz.go	69.92% <0.00%> (-5.14%)	⬇️
cmd/run.go	88.34% <0.00%> (-5.06%)	⬇️
app/eth2wrap/eth2wrap.go	51.87% <0.00%> (-4.59%)	⬇️
app/eth2wrap/httpwrap.go	22.76% <0.00%> (-2.70%)	⬇️
app/monitoringapi.go	80.79% <0.00%> (-2.68%)	⬇️
cluster/lock.go	56.41% <0.00%> (-1.70%)	⬇️
eth2util/deposit/deposit.go	54.26% <0.00%> (-1.65%)	⬇️
... and 40 more

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