[ML] AIOps Log Rate Analysis: 8.12 improvements.

[walterra]

Follow up to #163690.

API

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1. Don't re-add match_all queries when constructing analysis queries. [ML] AIOps Log Rate Analysis: Improve query generators #171008
2. Fix cases where we add range for @timestamp multiple times. [ML] AIOps Log Rate Analysis: Improve query generators #171008

Analysis

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1. [ML] AIOps Log Rate Analysis: Show top N results when no docs in baseline or deviation. #169767
   :ml Feature:ML/AIOps v8.12.0 +3
   

Tech debt

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1. The route file log_rate_analysis.ts is now 800+ lines and it gets pretty hard to follow what the stream is doing and returning. We should refactor and split up the file into parts of the analysis. [ML] AIOps: Log rate analysis route refactor #169660
2. SignificantTerm should be renamed into SignificantItem because it now covers both terms and text field based categories. [ML] AIOps Log Rate Analysis: Rename SignificantTerm to SignificantItem. #169756

[elasticmachine]

Pinging @elastic/ml-ui (:ml)

[peteharverson]

Created a follow up issue for 8.13 here: #172981. Closing this one.