Improve metrics library to enforce best practices

[lambdanis]

There were many changes made recently to improve how Prometheus metrics are defined and managed in Tetragon:

• cleanup metrics for deleted pods to prevent growing cardinality: Delete metrics for deleted pods #1279
• labels configuration for high-cardinality metrics: metrics: Add metrics label filter configuration #1444 and follow-up refactorings Refactor and extend the interface for metrics with configurable labels #1548, metrics: Refactor metrics label filter logic #2321 and metrics: Refactor metrics label filter logic #2373
• expose metrics directly from BPF maps: Define map metrics as custom collectors #1510 (and a few PRs using helpers introduced there)
• initialize metrics with labels for predictable resources usage and easier queries: Initialize metrics with labels #2162
• autogenerated metrics docs and grouping metrics by function: Generate metrics reference docs #2164
• multiple fixes to individual metrics

Metrics now seem to be in a decent place. However, it's not intuitive for developers how to define them. Things like labels configuration, initialization and separate helpers for docs can be confusing.

The goal of this issue is to extend pkg/metrics library to provide an intuitive interface for defining metrics following best practices. Ideally we should also write dev docs and add metrics linting to CI.

[lambdanis]

Here's the proposal for Tetragon metrics framework: https://docs.google.com/document/d/1oP0hZ_yKHqflhRJdpIzaoIUzniCrKQz6_L1sxw1JVYA/edit?usp=sharing

The library part is merged: #2606

The next steps are refactoring existing metrics to fully use it. This is tracked in a dedicated project: https://github.com/orgs/cilium/projects/57