Attribute extraction callbacks for telemetry events.

ReqAI.generate/2 emits [:req_ai, :generate, :start], [:req_ai, :generate, :stop], and [:req_ai, :generate, :exception] events. ReqAI.stream/4 emits the same lifecycle events under [:req_ai, :stream]. Attach handlers with :telemetry to observe calls.

The provider module is the default attribute extractor and may implement any of this module's optional callbacks. Pass telemetry: MyApp.Telemetry to ReqAI.Provider.new/2 to select another extractor, or telemetry: false to disable emission. A :telemetry_metadata map adds static tags to events:

ReqAI.Provider.new(ReqAI.Provider.OpenAI,
  telemetry_metadata: %{feature: :summarizer}
)

Metadata uses simple atom keys: :operation, :provider, :model, :response_id, :response_model, :finish_reasons, :input_tokens, and :output_tokens. Optional usage breakdowns are :cache_read_input_tokens, :cache_write_input_tokens, and :reasoning_output_tokens. Input totals include cache reads/writes; output totals include reasoning. Breakdowns are subsets of these totals. Missing values are omitted, not recorded as zero. Extraction coverage varies by provider and request mode; attributes are included when available. Streaming calls also include stream: true. Stop metadata includes :http_status_code when an HTTP response is available, :error, and :error_type on errors.

Stop measurements include :duration. Streaming stop measurements also include :time_to_first_chunk when a chunk was received, including a protocol-only event. Both measurements use native time units. Applications integrating with OpenTelemetry can map metadata to its semantic conventions in their handlers.

Built-in extractors select operational attributes rather than recording request or response bodies. Custom extractors and static metadata control any additional data recorded. Exception events include the standard :telemetry.span/3 exception metadata, which may contain exception details.