serve metrics in clusternet-agent

[xieydd]

What type of PR is this?

kind/feature

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #

Special notes for your reviewer:

[codecov-commenter]

Codecov Report

Merging #608 (327fbe6) into main (e2ffa5f) will decrease coverage by 0.41%.
The diff coverage is n/a.

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@@            Coverage Diff             @@
##             main     #608      +/-   ##
==========================================
- Coverage   13.60%   13.19%   -0.41%     
==========================================
  Files          66       71       +5     
  Lines        7492     7792     +300     
==========================================
+ Hits         1019     1028       +9     
- Misses       6391     6681     +290     
- Partials       82       83       +1     

Impacted Files	Coverage Δ
...ernet/clusternet/pkg/hub/apiserver/shadow/utils.go	89.28% <0.00%> (-10.72%)	⬇️
...m/clusternet/clusternet/pkg/scheduler/scheduler.go	3.82% <0.00%> (-0.12%)	⬇️
...ithub.com/clusternet/clusternet/pkg/agent/agent.go	1.42% <0.00%> (-0.07%)	⬇️
...et/clusternet/pkg/registry/shadow/template/rest.go	2.87% <0.00%> (-0.01%)	⬇️
...hub.com/clusternet/clusternet/pkg/utils/options.go	0.00% <0.00%> (ø)
.../clusternet/clusternet/pkg/agent/status_manager.go	0.00% <0.00%> (ø)
...t/clusternet/pkg/registry/shadow/template/scale.go	0.00% <0.00%> (ø)
...github.com/clusternet/clusternet/pkg/utils/flag.go
...thub.com/clusternet/clusternet/pkg/agent/option.go
...ternet/clusternet/pkg/scheduler/apis/validation.go	42.85% <0.00%> (ø)
... and 6 more

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[dixudx]

Thanks for the work.