Extract reusable classes from flow metrics

[andsel]

Release notes

[rn:skip]

What does this PR do?

Extract common code from datapoint that can be used by the RetentionWindow to be used to compute a flow metric.
Moves RetentionWindow ExtendedFlowMetric's inner class to be standalone class usable by other metric implementations.

Why is it important/What is the impact to the user?

As a developer of histogram flow metric I want to reuse existing code used to implement ExtendedFlowMetric.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] I have made corresponding change to the default configuration files (and/or docker env variables)
• [ ] I have added tests that prove my fix is effective or that my feature works

Author's Checklist

• [ ]

How to test this PR locally

Related issues

• Closes refactor flow metrics to being able to handle also histograms and not just scalar values. #17974

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[mergify]

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[Copilot]

Pull request overview

This PR extracts reusable classes from the ExtendedFlowMetric implementation to enable future support for histogram flow metrics. The refactoring moves the RetentionWindow inner class to a standalone class and creates a new DatapointCapture base class to generalize capture types beyond just FlowCapture.

Key changes:

• Introduced DatapointCapture as an abstract base class for point-in-time metric data with timing information
• Refactored FlowCapture to extend DatapointCapture, moving common nanoTime handling to the base class
• Extracted RetentionWindow from an inner class to a standalone package-private class that works with DatapointCapture instead of just FlowCapture

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 3 comments.

File	Description
DatapointCapture.java	New abstract base class providing timing information and capture selection logic for all datapoint types
FlowCapture.java	Refactored to extend DatapointCapture, removing duplicated nanoTime field and method
RetentionWindow.java	Extracted from ExtendedFlowMetric as a standalone class, generalized to work with DatapointCapture instead of FlowCapture
ExtendedFlowMetric.java	Removed inner RetentionWindow class and selectNewerCapture utility method, added cast when consuming baseline captures

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[Copilot]

The baseline() method now returns Optional, but this code unconditionally casts it to FlowCapture. While this may work in the current implementation since RetentionWindow is only used with FlowCapture instances, this cast could fail at runtime if RetentionWindow is later used with other DatapointCapture subtypes. Consider either: 1) making RetentionWindow generic over the capture type, or 2) adding a type check before the cast with appropriate error handling.

Suggested change

	.map((baseline) -> calculateRate(currentCapture, (FlowCapture) baseline))
	.orElseGet(OptionalDouble::empty)
	.ifPresent((rate) -> rates.put(window.policy.policyName(), rate)));
	.flatMap(baseline -> {
	if (baseline instanceof FlowCapture) {
	return calculateRate(currentCapture, (FlowCapture) baseline);
	} else {
	LOGGER.warn("RetentionWindow baseline is not a FlowCapture for policy '{}'; skipping rate calculation", window.policy.policyName());
	return OptionalDouble.empty();
	}
	})
	.ifPresent(rate -> rates.put(window.policy.policyName(), rate)));

[Copilot]

Pull request overview

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

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[elasticmachine]

💚 Build Succeeded

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• Commit: 77c8b7a

History

• 💔 Build #4028 failed b0b0721
• 💛 Build #3976 was flaky ad3f56f

cc @andsel