chore(metanode): only delete the metrics of volumes that no longer exist on the metanode

[daimashusheng]

…e deleted to avoid broken lines in Grafana

Motivation

If we delete the metrics of volumes that still exist on the metanode, Grafana may show broken lines in the charts.

Modifications

We only delete the metrics of volumes that no longer exist on the metanode.

Types of changes

• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] Documentation Update (if none of the other choices apply)
• [x] So on...

Verifying this change

• [x] Make sure that the change passes the testing checks.

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

• [ ] Master
• [x] MetaNode
• [ ] DataNode
• [ ] ObjectNode
• [ ] AuthNode
• [ ] LcNode
• [ ] Blobstore
• [ ] Client
• [ ] Cli
• [ ] SDK
• [ ] Other Tools
• [ ] Common Packages
• [ ] Dependencies
• [ ] Anything that affects deployment

Documentation

• [ ] doc
• [ ] doc-required
• [ ] doc-not-needed
• [ ] doc-complete

Review Expection

• [ ] in-two-days
• [ ] weekly
• [ ] free-time
• [ ] whenever

Matching PR in forked repository

PR in forked repository:

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[codecov[bot]]

Codecov Report

:x: Patch coverage is 0% with 17 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 23.14%. Comparing base (8960fc7) to head (ab5a0fc). :warning: Report is 9 commits behind head on master.

Files with missing lines	Patch %	Lines
metanode/metrics.go	0.00%	17 Missing :warning:

Additional details and impacted files

@@             Coverage Diff             @@
##           master    #3967       +/-   ##
===========================================
- Coverage   56.83%   23.14%   -33.69%     
===========================================
  Files         431      386       -45     
  Lines       88835   130172    +41337     
===========================================
- Hits        50491    30131    -20360     
- Misses      34731    96348    +61617     
- Partials     3613     3693       +80     

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