Builds histogram records for the four OpenTelemetry GenAI client metrics: gen_ai.client.operation.duration, gen_ai.client.token.usage, gen_ai.client.operation.time_to_first_chunk, and gen_ai.client.operation.time_per_output_chunk.

Shared between ReqLLM.OpenTelemetry (which feeds them to an OTel meter) and ReqLLM.Telemetry.OpenTelemetry (which returns them in the span stub).

stop/2 returns a list of records like:

%{
  name: "gen_ai.client.operation.duration",
  value: 0.412,
  unit: "s",
  description: "GenAI operation duration.",
  boundaries: [0.01, 0.02, 0.04, ...],
  attributes: %{
    "gen_ai.operation.name" => "chat",
    "gen_ai.provider.name" => "openai",
    "gen_ai.request.model" => "gpt-5",
    "gen_ai.response.model" => "gpt-5-2025-04-01",
    "server.address" => "api.openai.com",
    "server.port" => 443
  }
}

TTFC and TPOC records are only emitted for mode: :stream requests that observed at least one non-empty content chunk. Token histograms emit on :stop only; exception/2 emits the duration record with error.type populated so failures stay visible in latency charts.

Bucket boundaries

The bucket boundaries on each record (@duration_boundaries, @token_boundaries) are mandated by the OpenTelemetry GenAI metrics spec, not chosen by ReqLLM. Backends like Prometheus need fixed boundaries baked into the instrument at creation time, and the spec defines them up-front so different GenAI clients produce histograms a dashboard can compare apples-to-apples.

The two scales reflect what LLM workloads actually look like:

• Durations double from 10 ms up to ~82 s ([0.01, 0.02, 0.04, …, 81.92]) — short embeddings calls and long reasoning streams both fit in the same histogram with useful resolution.
• Token counts quadruple from 1 up to ~67 M ([1, 4, 16, …, 67_108_864]) — single-token completions and multi-million-token context windows both stay on-scale.

Exposed via duration_boundaries/0 and token_boundaries/0 for hosts that wire up custom histogram instruments themselves.