Mod: update the comments in metrics

[williamfeng323]

What this PR does:
Mod: Updated the comments in metrics, corrected the grammar.

[codecov-commenter]

Codecov Report

Merging #547 into develop will decrease coverage by 0.69%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #547      +/-   ##
===========================================
- Coverage    67.20%   66.51%   -0.70%     
===========================================
  Files          174      184      +10     
  Lines         9261     9712     +451     
===========================================
+ Hits          6224     6460     +236     
- Misses        2432     2611     +179     
- Partials       605      641      +36     

Impacted Files	Coverage Δ
metrics/prometheus/reporter.go	100.00% <100.00%> (ø)
registry/zookeeper/registry.go	46.15% <0.00%> (-9.30%)	⬇️
protocol/dubbo/listener.go	57.52% <0.00%> (-5.38%)	⬇️
common/url.go	65.83% <0.00%> (-5.33%)	⬇️
config_center/nacos/impl.go	71.08% <0.00%> (-4.31%)	⬇️
config_center/apollo/impl.go	82.71% <0.00%> (-4.30%)	⬇️
remoting/zookeeper/facade.go	70.27% <0.00%> (-4.02%)	⬇️
config/provider_config.go	55.88% <0.00%> (-3.50%)	⬇️
filter/filter_impl/hystrix_filter.go	68.64% <0.00%> (-3.39%)	⬇️
registry/nacos/registry.go	58.42% <0.00%> (-3.37%)	⬇️
... and 64 more

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