No-op Meter implementation (OTel metrics/noop.md §Meter, Status: Stable, L84-L266).

Implements the Otel.API.Metrics.Meter behaviour with the minimum work required by spec: instrument creation returns an Otel.API.Metrics.Instrument struct populated with identifying fields and a back-reference to this module; recording operations return :ok; callback registration returns an opaque {Noop, :noop} handle; unregistration returns :ok; enabled?/2 returns false.

Activated automatically by Otel.API.Metrics.MeterProvider when no SDK is installed (default fallback {Otel.API.Metrics.Meter.Noop, []}).

Noop MUSTs from metrics/noop.md

The following invariants hold for this module:

• No state (L86-L88) — no module attributes, no process state
• No errors or logs (L90-L91) — every function returns a well-formed value; no Logger calls
• Safe for concurrent use (L93) — stateless
• Accept all creation params without validation (L103-L107, L117-L121, L131-L135, L145-L149, L160-L164, L175-L179) — build/4 stores whatever the caller provides
• No callback retention (L149, L164, L179) — the /5 arities of async creation capture _callback and _callback_args with _ prefixes; neither is stored on the returned instrument
• No observation retention (L240, L253, L266) — the Noop has no callback-evaluation path, so there is nothing to retain

noop.md L95-L179 enumerates Counter, UpDownCounter, Histogram, Async Counter, Async UpDownCounter, and Async Gauge. Sync Gauge was added to api.md later and noop.md has not yet been updated; create_gauge/3 follows the same pattern as the other create_*/3 functions and is covered by the general §Meter invariants (L86-L93).

Divergences from opentelemetry-erlang

opentelemetry-erlang's otel_meter_noop.erl diverges in four places. All are spec-aligned:

1. Creation API shape — erlang exposes a single generic create_instrument/4,6 taking Kind as a parameter; we expose kind-specific callbacks (create_counter/3, create_observable_gauge/5, etc.) matching the Otel.API.Metrics.Meter behaviour.
2. register_callback return value — erlang returns ok. We return {__MODULE__, :noop} as the registration handle — keeps the shape consistent with the SDK-installed path, so unregister_callback/1 has a uniform input.
3. unregister_callback/1 — erlang has no such function. We implement it per spec api.md L419-L420 MUST ("user MUST be able to undo registration of the specific callback").
4. enabled?/2 — erlang has no such function. We implement it per spec api.md L475-L495 SHOULD and return false — signals to instrumentation authors that recording paths can be skipped while the Noop is active.

Defensive || fallbacks in build/4

Optional opts fields are coerced through || so the returned Instrument struct has well-formed values even when the caller passes an explicit nil:

• unit, description → || ""
• advisory → || []

This aligns with noop.md L81 "MUST accept these parameters. However, the MeterProvider MUST NOT validate any argument it receives" — acceptance over validation for the explicitly-optional fields.

name is a required positional parameter (String.t()) and is passed through unchanged; per .claude/rules/code-conventions.md §Happy-path only, the caller supplies a valid string and we do not defend against type violations at runtime.

Public API

References

• OTel Metrics Noop: opentelemetry-specification/specification/metrics/noop.md
• OTel Metrics API §Asynchronous Instrument API: opentelemetry-specification/specification/metrics/api.md L350-L472
• OTel Metrics API §Enabled: opentelemetry-specification/specification/metrics/api.md L475-L495
• Reference impl: opentelemetry-erlang/apps/opentelemetry_api_experimental/src/otel_meter_noop.erl