fix: time remaining should use better estimates

[rkuris]

Why this should be merged

This code does a much better job of estimating completion time, which improves my experience using avalanchego for bootstrapping.

How this works

Correctly computing an estimate involves:

• Estimating high early on, but gradually removing the fuzz as the load progresses. This code uses a 20% margin for this.
• Using a rolling window of samples to see what the rate is. This prevents a few large or slow blocks from making a big impact on the estimate right way. This code uses a window size of 10.
• Don't report estimates until we have some reasonable number of samples. Estimates will not be reported for about a minute with this code (10 samples at a minimum of 5s apart)
• Include a percentage complete. I found myself running this calculation all the time and it's nice to see how far along it is as a percentage.

How this was tested

Too many bootstraps. The 20% margin is a wild guess, but I wrote some code to show how much fuzz makes sense, and settled on 10 samples. It was not tested on a very slow machine.

Need to be documented in RELEASES.md?

Not a bad idea.

[Copilot]

Pull Request Overview

This PR improves the ETA estimation for bootstrapping operations by replacing the simple time-based calculation with a more sophisticated rolling window approach that provides better accuracy and user experience.

• Introduces a new EtaTracker struct that uses a rolling window of samples and applies a slowdown factor for more accurate time estimates
• Replaces direct EstimateETA calls with the new tracker in both bootstrap execution and fetching phases
• Adds percentage completion reporting alongside ETA estimates

Reviewed Changes

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

File	Description
utils/timer/eta.go	Implements new EtaTracker with rolling window sampling and slowdown factor for improved ETA accuracy
utils/timer/eta_test.go	Adds comprehensive test coverage for the new ETA tracking functionality
snow/engine/snowman/bootstrap/storage.go	Integrates EtaTracker for block execution progress with percentage reporting
snow/engine/snowman/bootstrap/bootstrapper.go	Replaces simple ETA calculation with EtaTracker for block fetching progress

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}

[Copilot]

The field name 'lowestSample' is misleading since it's initialized to the maximum value and tracks the minimum completed value. Consider renaming to 'minCompleted' or 'baselineCompleted' for clarity.

Suggested change

	lowestSample: math.MaxUint64, // Initialize to maximum value
	minCompleted: math.MaxUint64, // Initialize to maximum value

[rkuris]

hmm, I think it's the lowestSample. Perhaps it's also minCompleted but lowestSample better represents how it is used in the code.

[rkuris]

stupid go question: is there a better way to code this? I am only using this later to point to it.