Feat: add comments for config

[watermelo]

What this PR does:
Add comments for config

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[watermelo]

#566

[codecov-commenter]

Codecov Report

Merging #579 into develop will increase coverage by 0.09%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #579      +/-   ##
===========================================
+ Coverage    66.43%   66.52%   +0.09%     
===========================================
  Files          184      184              
  Lines         9712     9709       -3     
===========================================
+ Hits          6452     6459       +7     
+ Misses        2620     2606      -14     
- Partials       640      644       +4     

Impacted Files	Coverage Δ
config/application_config.go	55.55% <ø> (+13.88%)	⬆️
config/base_config.go	64.14% <ø> (ø)
config/config_center_config.go	71.42% <ø> (ø)
config/consumer_config.go	56.25% <ø> (ø)
config/generic_service.go	0.00% <ø> (ø)
config/graceful_shutdown.go	58.62% <ø> (ø)
config/graceful_shutdown_config.go	100.00% <ø> (ø)
config/metadata_report_config.go	48.71% <ø> (ø)
config/method_config.go	54.54% <ø> (ø)
config/mock_rpcservice.go	33.33% <ø> (ø)
... and 10 more

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[AlexStocks]

LGTM

[pantianying]

LGTM