Monitoring for Elasticsearch incremental updates buffer queue

What does this MR do?

This metric is used to track the size of updates that need to be synchronized to Elasticsearch. Since this is a custom queue and we're not using Sidekiq we needed to implement our own custom monitoring for it.

This MR just sets a gauge every minute in a cron worker which will ultimately be available in Prometheus. Later we will build a chart for this.

See #207095 (closed)

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

#207095 (closed)