balancer/least_request : Fix panic while handling resolver errors

[atollena]

There was a leftover embedded balancer in least request that was never set in the code when it got refactored to pick first as leaf. As a result, method calls forwarded to the embedded balancer would panic.

Remove the embedded balance and properly forward the missing methods.

Fixes #8332

RELEASE NOTES:

• balancer/least_request: fix panic on resolver errors when using least_request.

[codecov]

Codecov Report

Attention: Patch coverage is 44.44444% with 5 lines in your changes missing coverage. Please review.

Project coverage is 82.32%. Comparing base (f2d3e11) to head (e57a5de).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
balancer/leastrequest/leastrequest.go	44.44%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8333      +/-   ##
==========================================
- Coverage   82.41%   82.32%   -0.10%     
==========================================
  Files         419      419              
  Lines       42025    42034       +9     
==========================================
- Hits        34637    34604      -33     
- Misses       5944     5974      +30     
- Partials     1444     1456      +12     

Files with missing lines	Coverage Δ
balancer/leastrequest/leastrequest.go	85.24% <44.44%> (-3.25%)	⬇️

... and 21 files with indirect coverage changes

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[atollena]

Looking at the coverage report, I'm unclear on why the test doesn't cover the added implementation... Will look into it. Good to review otherwise.

[arjan-bal]

LGTM, thank you for the fix.