runtime/metrics: /gc/scan* metrics return zero [1.21 backport]

[gopherbot]

@mknyszek requested issue #62477 to be considered for backport to the next 1.21 minor release.

@gopherbot please open a backport issue for Go 1.21.

This issue means that some new metrics just don't work, and there's no workaround. Although the severity of the issue is quite low, the fix is also very safe and includes a test.

[gopherbot]

Change https://go.dev/cl/526116 mentions this issue: [release-branch.go1.21] runtime/metrics: fix /gc/scan/* metrics

[cherrymui]

Approved. Looks pretty safe.

[gopherbot]

Closed by merging cd66ca0 to release-branch.go1.21.