grpclog: refactor existing logger interfaces for better interoparability

[clement2026]

Fixes: #6044

What's changed:

• Implement fun V(l int) bool for PrefixLogger, and delegate calls to its logger field.

@easwars Just let me know if I misunderstood anything or if there’s more to do.

RELEASE NOTES: none

[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 81.58%. Comparing base (2bcbcab) to head (b82b049).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #7431      +/-   ##
==========================================
+ Coverage   81.52%   81.58%   +0.05%     
==========================================
  Files         350      350              
  Lines       26879    26886       +7     
==========================================
+ Hits        21913    21934      +21     
+ Misses       3772     3765       -7     
+ Partials     1194     1187       -7     

Files	Coverage Δ
internal/grpclog/prefixLogger.go	95.83% <100.00%> (+0.37%)	⬆️
internal/grpclog/grpclog.go	73.07% <60.00%> (-3.12%)	⬇️

... and 22 files with indirect coverage changes

[easwars]

Upon reviewing your PR, I realized that we need a bunch of other changes as part of this refactor. So, I ended up sending #7465.

I'm closing this since the PR mentioned above performs a more broader refactor of the grpclog package.

Thanks for your contributions. And feel free to look at our "Help Wanted" issues if you are interested in making other contributions to our repo.