feat(opentelemetry_rescaled): add rescaling meter provider

[sandersaares]

Adds a new opentelemetry_rescaled crate: a meter provider that wraps an inner OpenTelemetry meter provider and transparently emits rescaled side-by-side "sidecar" instruments for instruments configured per scope.

Problem

Some consumers need the same metric in a different unit or scale (for example, a duration measured in seconds also exported in milliseconds). Producing this by hand means duplicating every recording site, keeping two instruments in sync, and leaking the second unit into instrument-facing code.

What it delivers

A wrapping meter provider that, for instruments selected per scope at build time, creates a second instrument whose measurements are the original values multiplied by a fixed factor. The rescaling is invisible to instrument users, who interact only with their original instrument; the inner provider sees two independently registered instruments.

let outer = RescaledMetrics::builder(inner)
    .scope("my_scope_name", |scope| {
        scope.rescale("http.client.request.duration", "http.client.request.duration.millis", "ms", 1000.0);
    })
    .build();

• Supports every synchronous and observable instrument kind.
• Scales histogram bucket boundaries by the same factor so buckets stay meaningful.
• Rounds and saturates when rescaling integer instruments.
• The sidecar inherits the source description and requires a new unit; multiplication is the only transform.
• Unconfigured scopes and instruments pass straight through to the inner provider with no wrapping cost.

See crates/opentelemetry_rescaled/docs/DESIGN.md for the architecture and design tenets.

[github-actions]

⚠️ Breaking Changes Detected

error: failed to retrieve local crate data from git revision

Caused by:
    0: failed to retrieve manifest file from git revision source
    1: possibly due to errors: [
         failed when reading /home/runner/work/oxidizer/oxidizer/target/semver-checks/git-origin_main/64f4b2d034eca1b900afb1df4b7545bf6c69f81b/scripts/crate-template/Cargo.toml: TOML parse error at line 9, column 26
         |
       9 | keywords = ["oxidizer", {{CRATE_KEYWORDS}}]
         |                          ^
       missing key for inline table element, expected key
       : TOML parse error at line 9, column 26
         |
       9 | keywords = ["oxidizer", {{CRATE_KEYWORDS}}]
         |                          ^
       missing key for inline table element, expected key
       ,
         failed to parse /home/runner/work/oxidizer/oxidizer/target/semver-checks/git-origin_main/64f4b2d034eca1b900afb1df4b7545bf6c69f81b/Cargo.toml: no `package` table,
       ]
    2: package `opentelemetry_rescaled` not found in /home/runner/work/oxidizer/oxidizer/target/semver-checks/git-origin_main/64f4b2d034eca1b900afb1df4b7545bf6c69f81b

Stack backtrace:
   0: anyhow::error::<impl anyhow::Error>::msg
   1: cargo_semver_checks::rustdoc_gen::RustdocFromProjectRoot::get_crate_source
   2: cargo_semver_checks::rustdoc_gen::StatefulRustdocGenerator<cargo_semver_checks::rustdoc_gen::CoupledState>::prepare_generator
   3: cargo_semver_checks::Check::check_release::{{closure}}
   4: cargo_semver_checks::Check::check_release
   5: cargo_semver_checks::exit_on_error
   6: cargo_semver_checks::main
   7: std::sys::backtrace::__rust_begin_short_backtrace
   8: main

If the breaking changes are intentional then everything is fine - this message is merely informative.

Remember to apply a version number bump with the correct severity when publishing a version with breaking changes (1.x.x -> 2.x.x or 0.1.x -> 0.2.x).

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.0%. Comparing base (b695afd) to head (732802b).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main     #544    +/-   ##
========================================
  Coverage   100.0%   100.0%            
========================================
  Files         354      359     +5     
  Lines       26897    27189   +292     
========================================
+ Hits        26897    27189   +292     

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[Copilot]

Pull request overview

Adds a new opentelemetry_rescaled crate that wraps an inner OpenTelemetry MeterProvider and transparently mirrors configured instruments into rescaled “sidecar” instruments (including histogram boundary scaling), with opt-in configuration per instrumentation scope.

Changes:

• Introduces RescaledMetrics + builder that wraps an inner MeterProvider and conditionally applies per-scope rescale rules.
• Implements instrument interception via a custom InstrumentProvider (sync fan-out + async dual-registration with scaling observers).
• Adds end-to-end integration coverage, design docs, example, and spelling dictionary updates for the new terminology.

Reviewed changes

Copilot reviewed 12 out of 13 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
crates/opentelemetry_rescaled/src/lib.rs	Crate entrypoint, docs, and public exports.
crates/opentelemetry_rescaled/src/provider.rs	RescaledMetrics provider wrapper + builder and scope rule wiring.
crates/opentelemetry_rescaled/src/config.rs	Scope-level configuration model and validation for rescale rules.
crates/opentelemetry_rescaled/src/instruments.rs	InstrumentProvider implementation: sync fan-out and async scaling observer registration.
crates/opentelemetry_rescaled/src/rescale.rs	Core value-rescaling trait with rounding/saturation semantics + unit tests.
crates/opentelemetry_rescaled/tests/integration.rs	End-to-end tests against SdkMeterProvider and in-memory exporter for all instrument kinds.
crates/opentelemetry_rescaled/examples/print_metrics.rs	Runnable example showing sidecar emission and pass-through behavior.
crates/opentelemetry_rescaled/docs/DESIGN.md	Architecture/design rationale and invariants for the crate.
crates/opentelemetry_rescaled/README.md	Generated README content for the new crate.
crates/opentelemetry_rescaled/Cargo.toml	New crate manifest, deps, and external-types allowlist.
Cargo.lock	Adds opentelemetry_rescaled to the workspace lockfile.
AGENTS.md	Codifies PR-description guidance in repo agent guidelines.
.spelling	Adds new crate-related words to the spelling dictionary.

---

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[Copilot]

Pull request overview

Copilot reviewed 13 out of 14 changed files in this pull request and generated 2 comments.

[ralfbiedert]

I'm not a fan of this for multiple reasons. If there is a need to record different metrics, I think different metrics should be recorded. I'd rather we have a system in observed or so where you can define 'metric buddies' or metric dependencies, than to hard code strings and ad-hoc conversion logic. Even if we needed to have this via hooks it should at least be somehow typesafe-ish w.r.t, observed.