tbs: Clean up metric registration in storage manager

[isaacaflores2]

Motivation/summary

I noticed I forgot to clean up the metric registration. It is no longer required since storage metrics are now synchronous.

How to test these changes

1. Unit Test
2. Calling the stats endpoint localhost:5066/stats

Related issues

Part of https://github.com/elastic/apm-server/issues/17104

[github-actions[bot]]

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[mergify[bot]]

This pull request does not have a backport label. Could you fix it @isaacaflores2? 🙏 To fixup this pull request, you need to add the backport labels for the needed branches, such as:

• backport-7.17 is the label to automatically backport to the 7.17 branch.
• backport-8./d is the label to automatically backport to the 8./d branch. /d is the digit.
• backport-9./d is the label to automatically backport to the 9./d branch. /d is the digit.
• backport-active-all is the label that automatically backports to all active branches.
• backport-active-8 is the label that automatically backports to all active minor branches for the 8 major.
• backport-active-9 is the label that automatically backports to all active minor branches for the 9 major.

[github-actions[bot]]

@mergifyio backport 9.0

[mergify[bot]]

backport 9.0

✅ Backports have been created

• #17311 [9.0] (backport #17290) tbs: Clean up metric registration in storage manager has been created for branch 9.0