Logstash Integration: Adds new metrics and bumps version

[rookuu]

What does this PR do?

• Adds new metrics (https://www.elastic.co/docs/reference/logstash/tuning-logstash)
  • pipeline metric: worker utilisation
  • pipeline metric: worker concurrency
  • pipeline metric: queue backpressure
  • pipeline metric: workers
  • pipeline metric: batch size
  • pipeline metric: batch delay
  • pipeline filter metric: worker utilisation
  • pipeline output metric: worker utilisation
• Bumps the logstash integration version to 1.2.1

Motivation

As referenced in the doc these metrics are critical for diagnosing and tuning logstash instances.

Review checklist

• PR has a meaningful title or PR has the no-changelog label attached
• Feature or bugfix has tests
• Git history is clean
• If PR impacts documentation, docs team has been notified or an issue has been opened on the documentation repo
• If this PR includes a log pipeline, please add a description describing the remappers and processors.

Additional Notes

Anything else we should know when reviewing?