Ottotom (OpenTelemetry to text OpenMetrics)

A Rust crate for exporting OpenTelemetry metrics in the OpenMetrics text format. This serves as a protobuf-free alternative to the discontinued opentelemetry-prometheus crate.

This implementation tries to follow the OpenTelemetry-to-OpenMetrics conversion spec, though it takes some liberties where it contradicts the OpenMetrics spec. Some edge cases and complex metrics setups may not be handled correctly. See Tracey spec tracking for how spec-compliance is tracked.

Features

• Conversion of opentelemetry-sdk metric data to OpenMetrics-compliant text.
• Ready-to-use Exporter to register in opentelemetry, outputs metrics in the OpenMetrics text format.

Cargo features: exporter (the OpenMetricsExporter) and tracing (log dropped metrics and exporter errors) are on by default; experimental adds settings that produce output outside the OpenMetrics spec.

experimental is exempt from this crate's semver guarantees: what it exposes may change or disappear in any release, including a patch release.

Versioning

ottotom's minor version tracks the opentelemetry and opentelemetry_sdk minor it is built against, because their metric data types appear in this crate's API. ottotom 0.33.x requires opentelemetry 0.33 — pick the ottotom minor that matches the OpenTelemetry version in your dependency tree.

The minimum supported Rust version is 1.88. Raising it is a breaking change.

Usage

use std::time::Duration;
use ottotom::exporter::OpenMetricsExporter;
use opentelemetry_sdk::metrics::PeriodicReader;
use opentelemetry_sdk::metrics::SdkMeterProvider;

pub fn init_openmetrics_exporter() -> OpenMetricsExporter {
    let exporter = OpenMetricsExporter::default();
    let reader = PeriodicReader::builder(exporter.clone())
        .with_interval(Duration::from_secs(1))
        .build();
    let meter_provider = SdkMeterProvider::builder()
        .with_reader(reader)
        .build();
    opentelemetry::global::set_meter_provider(meter_provider);
    exporter
}

let exporter = init_openmetrics_exporter();
// Retain the exporter in you app state. Register some opentelmetry meters and fill them with data.
// Later on (e.g. in a `/metrics` endpoint) read the current metrics:
let openmetrics = exporter.text();
println!("{}", openmetrics);

Tracey spec tracking

This project uses Tracey to track which requirements of its source specifications are implemented and tested. The configuration lives in .config/tracey/config.styx and tracks two specification documents:

• docs/spec/openmetrics.md, condensed from the OpenMetrics text format spec. Annotations use the om[...] prefix.
• docs/spec/conversion.md, condensed from the OTLP metric points to Prometheus conversion spec. Annotations use the c[...] prefix.

Spec requirements are referenced inline in the code via comment annotations:

// om[impl text.eof]        // this code implements a requirement
// om[verify text.eof]      // this test verifies a requirement
// c[related metadata.unit-suffix]  // loosely connected

Useful commands:

• mise run tracey-status coverage overview per spec/implementation.
• mise run tracey-validate validate annotation references and naming.
• tracey web interactive coverage dashboard.
• tracey uncovered / tracey untested list the remaining work.