[8.17](backport #42565) Add latency metrics for logstash async output

[mergify]

Same as #42537 but for Logstash

#37445 added latency metrics for the sync output but the async output is what's used by the default configuration

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This is an automatic backport of pull request #42565 done by Mergify.

[mergify]

Cherry-pick of accc5e1 has failed:

On branch mergify/bp/8.17/pr-42565
Your branch is up to date with 'origin/8.17'.

You are currently cherry-picking commit accc5e147.
  (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:   libbeat/outputs/logstash/async.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

[elasticmachine]

Pinging @elastic/elastic-agent-data-plane (Team:Elastic-Agent-Data-Plane)