chore: add actuator comments

[Holdonbei]

What this PR does:
add comments to actuator related code .
related: #112

[mosn-community-bot]

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[seeflood]

Thanks for your contribution!
To fix the CI error, please modify the title to "feature: xxxx" or "feat: xxx"

[codecov]

Codecov Report

Merging #479 (68b1928) into main (f46cc1f) will not change coverage.
The diff coverage is n/a.

❗ Current head 68b1928 differs from pull request most recent head fe4f4c8. Consider uploading reports for the commit fe4f4c8 to get more accurate results

@@           Coverage Diff           @@
##             main     #479   +/-   ##
=======================================
  Coverage   60.50%   60.50%           
=======================================
  Files         118      118           
  Lines        6357     6357           
=======================================
  Hits         3846     3846           
  Misses       2142     2142           
  Partials      369      369           

Impacted Files	Coverage Δ
pkg/actuator/actuator.go	100.00% <ø> (ø)
pkg/actuator/health/endpoint.go	77.50% <ø> (ø)
pkg/actuator/health/indicator.go	64.70% <ø> (ø)
pkg/actuator/info/contributor.go	0.00% <ø> (ø)
pkg/actuator/info/endpoint.go	75.00% <ø> (ø)

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[seeflood]

lgtm