feat(otel-metrics): POC — centralize OTLP metrics export via a shared libdatadog exporter

[mabdinur]

What

Proof of concept (draft): move the OTLP metrics export logic into libdatadog's libdd-otel-telemetry crate — which now exposes a Datadog OTLP PushMetricExporter (DatadogMetricExporter, owning encode + transport + retry + attempt/success/failure telemetry counting). dd-trace-rs keeps its own SdkMeterProvider + PeriodicReader + resource + views and just plugs the exporter in.

Companion POC PRs: libdatadog DataDog/libdatadog#2292 (the crate + shared exporter), dd-trace-py DataDog/dd-trace-py#19354 (Python metrics, primitives aggregator).

Why this shape (not the primitives aggregator)

The primitives-only aggregator exists for FFI — Python/Node/Ruby/PHP can't pass SDK objects across the language boundary. dd-trace-rs is Rust-to-Rust in-process and keeps opentelemetry_sdk regardless (its trace pipeline needs it), so a primitives bridge there is pure overhead — an earlier revision hand-reimplemented ~500 lines of the SDK's own metrics API for no benefit. This approach centralizes only the export path (the part actually worth sharing) and is a net reduction in dd-trace-rs.

Two doors over one shared export core: FFI languages use the primitives aggregator; Rust plugs in the shared PushMetricExporter. The cost is that the crate exposes an SDK type on its Rust door and tracks the consumer's opentelemetry version (bumped 0.31→0.32) — a deliberate, Rust-only exception to "no SDK objects cross the boundary," invisible to the FFI consumers.

Changes

• metrics_reader.rs: builds the exporter via libdd_otel_telemetry::build_datadog_metric_exporter(config, temporality) and feeds it to a PeriodicReader; returns SdkMeterProvider as before. Views keep working.
• Deletes telemetry_metrics_exporter.rs — the attempt/success/failure counting now lives in the shared exporter; DatadogMetricExporter::counters() exposes a snapshot (wiring it into DD telemetry is a TODO).
• metrics features no longer pull opentelemetry-otlp (owned by libdatadog); opentelemetry_sdk stays for traces + the provider/reader.

Regression tests (existing suite, run as-is)

• integration_tests::metrics: 12 passed, 0 failed, 0 ignored (all view tests included).
• lib metrics_tests: 2 passed, 0 failed, 0 ignored.
• clippy -D warnings + fmt clean.

Net source delta vs main: +64 / −124.

POC caveats

• Export-telemetry counting is available via DatadogMetricExporter::counters() but not yet wired into DD's telemetry client (TODO).
• Temporary dependency pinning: libdd-otel-telemetry is unpublished, so it's a git dep + a minimal [patch.crates-io] for the 4 libdatadog crates it shares with dd-trace-rs. Revert once published.
• Metrics only — logs/traces untouched.

🤖 Generated with Claude Code

[datadog-prod-us1-5]

🎉 All green!

❄️ No new flaky tests detected

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