Update coverage for nativelink-metric-collector

[leodziki]

Description

Update coverage for metric-collector. Here some private ones are converted to public ones.(metrics_visitors.rs) Bazel test, cargo test, pre-commit all passed Fixes #1401

Type of change

• [x] New feature (non-breaking change which adds functionality)
• [x] This change requires a documentation update

  How Has This Been Tested?

  bazel test, cargo test

  Checklist

• [x] Updated documentation if needed
• [x] Tests added/amended
• [x] bazel test //... passes locally

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This change is 

[leodziki]

Asking review on this. @aaronmondal @MarcusSorealheis