Mod: add comment for metadata

[watermelo]

What this PR does:
add comment for metadata

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

[codecov-commenter]

Codecov Report

Merging #588 into develop will increase coverage by 1.10%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           develop     #588      +/-   ##
===========================================
+ Coverage    66.43%   67.53%   +1.10%     
===========================================
  Files          184      188       +4     
  Lines         9712     9852     +140     
===========================================
+ Hits          6452     6654     +202     
+ Misses        2620     2546      -74     
- Partials       640      652      +12     

Impacted Files	Coverage Δ
cluster/directory/base_directory.go	56.81% <0.00%> (-9.10%)	⬇️
registry/kubernetes/registry.go	67.30% <0.00%> (-3.85%)	⬇️
remoting/kubernetes/listener.go	52.83% <0.00%> (-3.25%)	⬇️
cluster/cluster_impl/base_cluster_invoker.go	70.83% <0.00%> (-1.64%)	⬇️
cluster/cluster_impl/failover_cluster_invoker.go	71.15% <0.00%> (-1.19%)	⬇️
cluster/cluster_impl/forking_cluster_invoker.go	58.33% <0.00%> (-0.21%)	⬇️
config/base_config.go	64.14% <0.00%> (ø)
config/graceful_shutdown.go	58.62% <0.00%> (ø)
cluster/loadbalance/random.go	100.00% <0.00%> (ø)
config/config_center_config.go	71.42% <0.00%> (ø)
... and 38 more

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[watermelo]

#570