ExAgent.Observability.ReqLLM.Metrics (ExAgent v2.2.2)

Optional histogram adapter for the published OTel experimental API 0.6.

Enable through ExAgent.Observability.ReqLLM.attach(metrics: [models: ["my-model"]]). The application supplies and starts its metric SDK, reader and exporter. This module neither starts them nor adds another telemetry handler. Metrics default to off. Models outside the host's fixed allowlist become "other"; the only other dimensions are input/output token type and a fixed "error" label. Request IDs, endpoints, response model names and content are never dimensions. Token measurements retain ReqLLM's normalized accounting quality.

Uses public get_meter/1, lookup_instrument/2, create_histogram/3 and record/5. No instrument cache survives an SDK restart. Missing/stopped SDKs drop metrics; failures in optional metrics never fail the request or detach tracing.