[ML] Explain Log Rate Spikes: Fix frequent_items agg config.

[walterra]

Summary

Part of #138117.

• Tweaks the frequent_items agg config. should_minimum_match:2 for the terms should clause improves getting back groups of results. minimum_set_size:2 will avoid single item frequent sets.
• Passes on a possible filter bar query to the frequent_items agg.

Checklist

• This was checked for breaking API changes and was labeled appropriately

[elasticmachine]

Pinging @elastic/ml-ui (:ml)

[kibana-ci]

💚 Build Succeeded

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• Commit: e7bf9b9

Metrics [docs]

✅ unchanged

History

• 💔 Build #72694 failed 5c24e69

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@elasticmachine merge upstream

cc @walterra

[peteharverson]

Code LGTM

[alvarezmelissa87]

LGTM ⚡