[8.19] (backport #17154) tbs: Update storage metrics to be reported synchronously in the existing `runDiskUsageLoop` method

[mergify[bot]]

Motivation/summary

Updated metrics for apm-server.sampling.tail.storage.lsm_size and apm-server.sampling.tail.storage.value_log_size to be synchronous so we can control the reporting interval. The existing runDropLoop is run every 1m.

Checklist

• [ ] Update CHANGELOG.asciidoc
• [ ] Documentation has been updated

For functional changes, consider:

• Is it observable through the addition of either logging or metrics?
• Is its use being published in telemetry to enable product improvement?
• Have system tests been added to avoid regression?

How to test these changes

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

    "sampling": {
    	"tail": {
    		"storage": {
    			"lsm_size": 7217,
    			"value_log_size": 6320
    		   }
    	   }
    }
   

Related issues

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

---

This is an automatic backport of pull request #17154 done by [Mergify](https://mergify.com).

[mergify[bot]]

Cherry-pick of d147b7af60aff2b58d9db41d4f01015386389b54 has failed:

On branch mergify/bp/8.19/pr-17154
Your branch is up to date with 'origin/8.19'.

You are currently cherry-picking commit d147b7af.
  (fix conflicts and run "git cherry-pick --continue")
  (use "git cherry-pick --skip" to skip this patch)
  (use "git cherry-pick --abort" to cancel the cherry-pick operation)

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   x-pack/apm-server/main_test.go
	both modified:   x-pack/apm-server/sampling/eventstorage/storage_manager.go
	both modified:   x-pack/apm-server/sampling/processor_test.go

no changes added to commit (use "git add" and/or "git commit -a")

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[github-actions[bot]]

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