Use flushing_duration_s for indexing SLI

What does this MR do and why?

Adds indexing apdex tracking to Global Search indexing.

Note: currently the search_indexing_duration_s is inaccurate and is being replaced by search_flushing_duration_s here. As a result, the content apdex target is only set based on indexing Wikis.

After this is merged, we can look at search_flushing_duration_s to have a more accurate apdex target.

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How to set up and validate locally

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MR acceptance checklist

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