Use Sampler instead of worker for monitoring Global Search queue

What does this MR do?

Introduced in !27384 (merged) we are tracking the size of the buffer queue used for incremental Elasticsearch updates. This was implemented as a worker that runs every minute but this has some problems as it runs on random sidekiq nodes and causes the data in Prometheus to appear as multiple unpredictable lines and there is not way to easily know which is the most up to date metric. For this reason it makes more sense to run it predictably and also on all sidekiq nodes so that even if one is down we'd still see the queue size growing.

This was suggested in gitlab-com/runbooks!2046 (comment 311761395)

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Does this MR meet the acceptance criteria?

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