Cherry-pick #8227 to v1.72.x

[arjan-bal]

Original PR: #8227
Related Issue: #8225

RELEASE NOTES:

• balancer/pickfirstleaf: Ignore the deprecated Address.Metadata field.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.09%. Comparing base (fd6f585) to head (c13ff0b).
Report is 1 commits behind head on v1.72.x.

Additional details and impacted files

@@             Coverage Diff             @@
##           v1.72.x    #8259      +/-   ##
===========================================
- Coverage    82.18%   82.09%   -0.09%     
===========================================
  Files          412      412              
  Lines        40346    40345       -1     
===========================================
- Hits         33159    33123      -36     
- Misses        5833     5864      +31     
- Partials      1354     1358       +4     

Files with missing lines	Coverage Δ
balancer/pickfirst/pickfirstleaf/pickfirstleaf.go	86.57% <100.00%> (-0.03%)	⬇️

... and 25 files with indirect coverage changes

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