xds: make least request available by default

[atollena]

RELEASE NOTES:

• xds: enable least request LB policy by default. It can be disabled by setting GRPC_EXPERIMENTAL_ENABLE_LEAST_REQUEST=false in your environment.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.11%. Comparing base (68205d5) to head (4d25427).
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8253      +/-   ##
==========================================
- Coverage   82.13%   82.11%   -0.03%     
==========================================
  Files         412      412              
  Lines       40487    40486       -1     
==========================================
- Hits        33256    33247       -9     
- Misses       5867     5873       +6     
- Partials     1364     1366       +2     

Files with missing lines	Coverage Δ
internal/envconfig/envconfig.go	100.00% <ø> (ø)
.../internal/xdsclient/xdslbregistry/xdslbregistry.go	90.47% <ø> (-0.83%)	⬇️

... and 25 files with indirect coverage changes

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[dfawley]

Thanks!

[dfawley]

How did you find this? :)

[atollena]

I actually ran into it while reading the code a while ago... Then I figured I'd grep for "least request" in this PR.