balancer: introduce env flag for case-sensitive registry

[Pranjali-2501]

Part of #5288

Follow up of PR: #6647

This PR introduces the GRPC_GO_EXPERIMENTAL_CASE_SENSITIVE_BALANCER_REGISTRIES environment variable to transition balancer registries to be case-sensitive.

Default Behavior:
* The env var is disabled by default.
* The registry remains case-insensitive (legacy behavior) and a warning is logged if a balancer name contains uppercase letters, notifying users of the upcoming change.

RELEASE NOTES:

• balancer: grpc will log a warning if balancer registries are used with uppercase letter(s).

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.99%. Comparing base (629ef39) to head (750ae1e).
⚠️ Report is 21 commits behind head on master.

Files with missing lines	Patch %	Lines
balancer/balancer.go	81.81%	0 Missing and 2 partials ⚠️
internal/envconfig/envconfig.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8837      +/-   ##
==========================================
- Coverage   83.22%   82.99%   -0.24%     
==========================================
  Files         417      414       -3     
  Lines       32920    32783     -137     
==========================================
- Hits        27397    27207     -190     
  Misses       4108     4108              
- Partials     1415     1468      +53     

Files with missing lines	Coverage Δ
internal/envconfig/envconfig.go	73.33% <0.00%> (-26.67%)	⬇️
balancer/balancer.go	66.66% <81.81%> (-27.09%)	⬇️

... and 21 files with indirect coverage changes

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[easwars]

I think a couple of things need to be done before we can make a change like this that can break our users:

• Add an environment variable that is disabled by default. Maybe something like GRPC_GO_EXPERIMENTAL_CASE_SENSITIVE_BALANCER_REGISTRIES. So, there is no behavior change.
• In a follow-up PR (after a release), we can enable the env var by default
• In a follow-up PR (after another release), we can get rid of the env var

Also check internal usages in google3 if there are balancers that need to be changed.

[easwars]

Please update the PR description and the title. Also, the related needs to stay open so that we do the follow-up activities (changing the env var to true by default, and getting rid of it).

[easwars]

@dfawley : Do you have a suggestion for the name of the env var? I suggested GRPC_GO_EXPERIMENTAL_CASE_SENSITIVE_BALANCER_REGISTRIES. But looking at it now, it seems very long, but I can't think of anything shorter that still conveys the message. Thanks.

[dfawley]

it seems very long

Maybe that's a feature? :)

I don't think the name for this is too important. We'll be deleting it (one way or another) after a few releases? And nobody should be relying on it for anything more than a workaround?

[Pranjali-2501]

I don't think the name for this is too important. We'll be deleting it (one way or another) after a few releases? And nobody should be relying on it for anything more than a workaround?

Yes, we will be deleting it after few releases.