feature: add gc disks metrics

[yeya24]

Signed-off-by: yeya24 [email protected]

Ⅰ. Describe what this PR did

Add metrics about disk gc

Ⅱ. Does this pull request fix one issue?

Fixes #958

Ⅲ. Why don't you add test cases (unit test/integration test)? (你真的觉得不需要加测试吗？)

I think we should consider opening a new PR for the whole tests of supernode gc.

Ⅳ. Describe how to verify it

Ⅴ. Special notes for reviews

[pouchrobot]

Thanks for your contribution. 🍻 @yeya24
While we thought PR TITLE could be more specific, longer than 20 chars.
Please edit this PR title instead of opening a new one.
More details, please refer to https://github.com/dragonflyoss/Dragonfly/blob/master/CONTRIBUTING.md

[codecov-io]

Codecov Report

Merging #966 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #966   +/-   ##
=======================================
  Coverage   46.48%   46.48%           
=======================================
  Files         116      116           
  Lines        6908     6908           
=======================================
  Hits         3211     3211           
  Misses       3440     3440           
  Partials      257      257

Impacted Files	Coverage Δ
supernode/config/config.go	72.91% <ø> (ø)	⬆️

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[starnop]

LGTM. And I think we should merge the PR #963 firstly. 😄

[starnop]

I think we should consider opening a new PR for the whole tests of supernode gc.

That's right. And could you please open an issue to record that? THX