balancer: remove grpc.lb.pick_first.* metrics

[deepgolani4]

Fixes #8752

The changes remove the three grpc.lb.pick_first.* metric registrations and their recording calls from pickfirst.go (along with the now-unneeded metricsRecorder/target fields), then update all tests and examples to assert on the equivalent grpc.subchannel.* metrics instead.

RELEASE NOTES:

• Remove grpc.lb.pick_first.* metrics.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: deepgolani4 / name: deepgolani4 (643ead2)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.25%. Comparing base (484f150) to head (ebca066).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #9215      +/-   ##
==========================================
+ Coverage   83.19%   83.25%   +0.05%     
==========================================
  Files         420      420              
  Lines       34010    34017       +7     
==========================================
+ Hits        28295    28320      +25     
+ Misses       4281     4265      -16     
+ Partials     1434     1432       -2     

Files with missing lines	Coverage Δ
balancer/pickfirst/pickfirst.go	90.07% <100.00%> (+0.31%)	⬆️

... and 26 files with indirect coverage changes

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