docs(metrics): prepare the new metrics crates for publishing

[ethanolchik]

Summary

• Add package metadata and READMEs for foundations-metrics and foundations-metrics-registry.
• Document migration from the foundations::telemetry::metrics facade, including custom metrics and removed legacy collectors.
• Demonstrate native histogram request latency in the HTTP server example.
• Document why the finite terminal histogram bound is translated to +Inf during text encoding.

Testing

• Full nextest and doctest suites passed.
• Strict clippy passed.
• Feature powerset check passed.
• HTTP server example check passed.
• Package-content checks passed for both metrics crates.