feat: add metrics-exporter-opentelemetry crate

Cargo.toml

@@ -4,6 +4,7 @@ members = [
   "metrics",
   "metrics-benchmark",
   "metrics-exporter-dogstatsd",
+  "metrics-exporter-opentelemetry",
   "metrics-exporter-prometheus",
   "metrics-exporter-tcp",
   "metrics-observer",
@@ -48,6 +49,8 @@ ndarray = { version = "0.16", default-features = false }
 ndarray-stats = { version = "0.6", default-features = false }
 noisy_float = { version = "0.2", default-features = false }
 once_cell = { version = "1", default-features = false, features = ["std"] }
+opentelemetry = { version = "0.30", default-features = false }
+opentelemetry_sdk = { version = "0.30", default-features = false }
 ordered-float = { version = "4.2", default-features = false }
 parking_lot = { version = "0.12", default-features = false }
 portable-atomic = { version = "1", default-features = false }
@@ -66,6 +69,7 @@ radix_trie = { version = "0.2", default-features = false }
 rand = { version = "0.9", default-features = false, features = [ "thread_rng" ] }
 rand_xoshiro = { version = "0.7", default-features = false }
 ratatui = { version = "0.28", default-features = false }
+scc = { version = "2.1", default-features = false }
 sketches-ddsketch = { version = "0.3", default-features = false }
 thiserror = { version = "2", default-features = false }
 tokio = { version = "1", default-features = false, features = ["rt", "net", "time", "rt-multi-thread"] }

metrics-exporter-opentelemetry/CHANGELOG.md

@@ -0,0 +1,13 @@
+# Changelog
+
+All notable changes to this project will be documented in this file.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+### Added
+- Initial release of OpenTelemetry metrics exporter for `metrics`
+- Support for counters, gauges, and histograms
+- Attribute/label support

metrics-exporter-opentelemetry/Cargo.toml

@@ -0,0 +1,30 @@
+[package]
+name = "metrics-exporter-opentelemetry"
+version = "0.1.0"
+edition = "2021"
+rust-version = "1.75.0"
+
+description = "A metrics-compatible exporter for sending metrics to OpenTelemetry collectors."
+license = { workspace = true }
+authors = ["Metrics Contributors"]
+repository = { workspace = true }
+homepage = { workspace = true }
+documentation = "https://docs.rs/metrics-exporter-opentelemetry"
+readme = "README.md"
+
+categories = ["development-tools::debugging"]
+keywords = ["metrics", "telemetry", "opentelemetry", "otel", "observability"]
+
+[dependencies]
+metrics = { version = "^0.24", path = "../metrics" }
+metrics-util = { version = "^0.20", path = "../metrics-util", default-features = false, features = ["registry"] }
+opentelemetry = { workspace = true, features = ["metrics"] }
+portable-atomic = { workspace = true, features = ["float", "critical-section"] }
+scc = { workspace = true }
+
+[dev-dependencies]
+opentelemetry_sdk = { workspace = true, features = ["metrics", "rt-tokio", "testing"] }
+
+[package.metadata.docs.rs]
+all-features = true
+rustdoc-args = ["--cfg", "docsrs"]

metrics-exporter-opentelemetry/README.md

@@ -0,0 +1,100 @@
+# metrics-exporter-opentelemetry
+
+[![docs-badge][docs-badge]][docs] [![crates-badge][crates-badge]][crates] [![license-badge][license-badge]][license]
+
+[docs-badge]: https://docs.rs/metrics-exporter-opentelemetry/badge.svg
+[docs]: https://docs.rs/metrics-exporter-opentelemetry
+[crates-badge]: https://img.shields.io/crates/v/metrics-exporter-opentelemetry.svg
+[crates]: https://crates.io/crates/metrics-exporter-opentelemetry
+[license-badge]: https://img.shields.io/crates/l/metrics-exporter-opentelemetry.svg
+[license]: #license
+
+A [`metrics`]-compatible exporter for sending metrics to OpenTelemetry collectors.
+
+[`metrics`]: https://docs.rs/metrics/
+
+## Overview
+
+A [`metrics`]-compatible exporter for OpenTelemetry collectors and OTLP endpoints.
+
+## Features
+
+- Counters, gauges, and histograms
+- Custom histogram bucket boundaries
+- Metric descriptions and units
+- Lock-free concurrent data structures
+- Works with any OpenTelemetry [`Meter`]
+
+[`Meter`]: https://docs.rs/opentelemetry/latest/opentelemetry/metrics/trait.Meter.html
+
+## Quick Start
+
+Add this to your `Cargo.toml`:
+
+```toml
+[dependencies]
+metrics = "0.24"
+metrics-exporter-opentelemetry = "0.1"
+opentelemetry = "0.30"
+opentelemetry_sdk = "0.30"
+```
+
+Basic usage:
+
+```rust
+use metrics_exporter_opentelemetry::OpenTelemetryRecorder;
+use opentelemetry::metrics::MeterProvider;
+use opentelemetry_sdk::metrics::SdkMeterProvider;
+
+// Create an OpenTelemetry meter
+let provider = SdkMeterProvider::default();
+let meter = provider.meter("my_application");
+
+// Create and install the recorder
+let recorder = OpenTelemetryRecorder::new(meter);
+metrics::set_global_recorder(recorder).expect("failed to install recorder");
+
+// Use metrics as normal
+metrics::counter!("requests_total", "method" => "GET").increment(1);
+metrics::gauge!("cpu_usage", "core" => "0").set(45.2);
+metrics::histogram!("response_time", "endpoint" => "/api/users").record(0.123);
+```
+
+## Custom Histogram Boundaries
+
+```rust
+let recorder = OpenTelemetryRecorder::new(meter);
+
+recorder.set_histogram_bounds(
+    &metrics::KeyName::from("response_time"),
+    vec![0.001, 0.005, 0.01, 0.05, 0.1, 0.25, 0.5, 1.0, 2.5, 5.0, 10.0]
+);
+
+metrics::set_global_recorder(recorder).expect("failed to install recorder");
+```
+
+## Metric Descriptions and Units
+
+You can provide descriptions and units for your metrics using the `describe_*` macros. 
+
+CAUTION: These macros must be called before the metrics are recorded. The instruments created before calling these macros will not have descriptions or units.
+
+```rust
+use metrics::{describe_counter, describe_histogram, Unit};
+
+describe_counter!("requests_total", Unit::Count, "Total HTTP requests");
+describe_histogram!("response_time", Unit::Seconds, "Response time distribution");
+
+metrics::counter!("requests_total").increment(1);
+metrics::histogram!("response_time").record(0.045);
+```
+
+## Compatibility
+
+### Metric Type Mapping
+
+| `metrics` Type | OpenTelemetry Instrument |
+|----------------|-------------------------|
+| `Counter` | `ObservableCounter` (u64) |
+| `Gauge` | `ObservableGauge` (f64) |
+| `Histogram` | `Histogram` (f64) |