spike: add HistoricMetric to capture current/1m/5m/15m/lifetime metric rates

[yaauie]

Release notes

[rn:skip]

What does this PR do?

Adds an HistoricMetric implementation for tracking metric rates (current/1m/5m/15m/lifetime), for eventual inclusion into the node stats API responses or for use in analyzing pipeline or plugin health.

This is a spike only, meant to convey intent, and will be carried forward to user-facing features in a future PR.

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

Currently, detecting event flow in a pipeline requires multiple captures of the node stats API and a lot of manual math. We would like to show flow rates to make analysis less manual and error prone. This is a single step in that direction.

Checklist

• [ ] My code follows the style guidelines of this project
• [x] 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

Use cases

Screenshots

Logs