feat(metrics-registry): introduce stable registry crate with protobuf model

[ethanolchik]

This pull request introduces a new crate, foundations-metrics-registry, to provide a stable, minimal core for Prometheus-compatible metrics serialisation in the foundations project. Currently, the crate only includes vendored Prometheus protobuf definitions, a build script to generate Rust types, and exposes these types for use by other crates. It also updates the workspace configuration and dependencies accordingly.

New crate: foundations-metrics-registry

• Added the foundations-metrics-registry crate to the workspace, with its own Cargo.toml and minimal dependencies, focused on stability and compatibility across major versions.

Protobuf integration

• Vendored the official Prometheus metrics.proto file in proto/metrics.proto, providing the canonical wire format for metrics serialisation.
• Added a build.rs script using protox and prost-build to generate Rust types from the vendored protobuf at build time.
• Exposed the generated types in src/proto/mod.rs, including a test verifying round-trip serialisation of the (native) histogram type.
• Included the generated code in src/proto/model.rs via a build-time include! macro.

Dependency and edition updates

• Updated workspace dependencies to include prost, prost-build, prost-types, and protox for protobuf code generation.

[TheJokr]

/bonk run cargo fmt and fix clippy lints that crop up in the "Clippy checks" CI job. amend the changes to the previous commit.

E: ah too bad, bonk refuses to push commits to PRs from forks :/

[ask-bonk]

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