Implementing Derivative using linear regression

[pabloem]

• Implements a one-pass simple linear regression algorithm for time-series (x-long, y-double)
• Implements per-type specializations that simply cast to double.
• Returns the slope of the linreg as the derivative

Implementation of https://prometheus.io/docs/prometheus/latest/querying/functions/#deriv

[elasticsearchmachine]

Pinging @elastic/es-storage-engine (Team:StorageEngine)

[dnhatn]

@pabloem Sorry for the delay. I left some comments. We can zoom if needed.

[github-actions]

🔍 Preview links for changed docs

• docs/reference/query-languages/esql/kibana/docs/functions/deriv.md
• docs/reference/query-languages/esql/functions-operators/time-series-aggregation-functions.md

[github-actions]

ℹ️ Important: Docs version tagging

👋 Thanks for updating the docs! Just a friendly reminder that our docs are now cumulative. This means all 9.x versions are documented on the same page and published off of the main branch, instead of creating separate pages for each minor version.

We use applies_to tags to mark version-specific features and changes.

Expand for a quick overview

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✅ At the page level to indicate which products/deployments the content applies to (mandatory)
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❌ Don't forget that applies_to tags can be used at the page, section, and inline level

🤔 Need help?

• Check out the cumulative docs guidelines
• Reach out in the #docs Slack channel

[dnhatn]

I've left some comments, but the PR looks great. Thanks Pablo!

[dnhatn]

LGTM - thanks for all iterations, @pabloem!