Add 0082 OTel error-span rendering and metric

[chris-colinsky]

Phase A Wave 2 of the v0.17.0 cycle. PR B of the 0082 split (structured-output failure diagnostics, spec v0.77.0): the OTel error-span rendering + the §11 token-usage-on-failure metric. PR A (merged, #229) landed the event + error surface; PR C (Langfuse failed-Generation rendering) is the last.

What

• The per-attempt LlmRetryAttemptEvent now carries the response-side surface on a structured_output_invalid failed attempt (mirroring PR A's terminal event).
• The bundled OTel observer renders it on the error openarmature.llm.complete span: a structured_output_invalid failure falls through the failed-attempt early-return to render finish_reason, usage, gated output.content, and the response identity, while keeping ERROR status + category. Every other failure category still ends there with no response.
• The opt-in openarmature.gen_ai.client.token.usage metric records the failure's usage (it already recorded whenever a usage record was present; a structured failure now carries one). No metric logic change.

Also fixed (observability §4.2)

The failed openarmature.llm.complete span carried ERROR status but no exception event. The node and invocation spans already recorded it via record_exception; the LLM span did not, a gap the 0082 error-span fixtures surfaced. It now records the OTel semconv exception event (exception.type / exception.message) on every failed attempt, matching the sibling spans, which also makes the enriched error_message locator load-bearing. See the CHANGELOG Fixed entry.

Conformance / tests

• Un-defers observability fixtures 124 (OTel error span, payload on/off) and 125 (token metric on the failure); only Langfuse 123 stays deferred.
• A dedicated failure-capable span-tree runner for 124, plus two OTel unit tests (surface rendered / output redacted under payload-off).
• conformance.toml keeps 0082 partial (only Langfuse remains).

Reviewed with a multi-agent adversarial pass. It caught a real blocker (the missing §4.2 exception event) that my initial span-tree runner was too weak to detect; the runner is strengthened to enforce status_description / attributes_absent / exception_recorded, and the observer records the event. Two stale-docstring findings folded in. Full suite green.

[Copilot]

Pull request overview

This PR advances proposal 0082 observability by ensuring a structured_output_invalid failed attempt carries enough response-side context to be rendered on the OTel openarmature.llm.complete error span (while preserving payload redaction behavior), and by un-deferring the corresponding conformance fixtures, including the §11 token-usage-on-failure metric fixture.

Changes:

• Extend LlmRetryAttemptEvent/OpenAI retry-attempt event construction so structured_output_invalid failures include response-side fields (finish_reason/usage/response id+model, plus payload-gated output content).
• Update the OTel observer to (a) record an OTel semconv exception event on failed LLM attempts and (b) render the response-side surface onto the error span for structured_output_invalid.
• Un-defer and strengthen observability conformance fixture coverage for the OTel error-span rendering (124) and token-usage-on-failure metric (125), plus add focused OTel unit tests.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
tests/unit/test_observability_otel.py	Adds unit tests asserting error-span surface rendering and payload redaction for structured-output failures.
tests/conformance/test_observability.py	Un-defers fixtures 124/125 and adds a dedicated runner + extra assertions for exception recording and payload redaction claims.
src/openarmature/observability/otel/observer.py	Records exception events on failed LLM attempts and renders response-side surface for structured_output_invalid error spans.
src/openarmature/llm/providers/openai.py	Includes response-side surface on LlmRetryAttemptEvent for StructuredOutputInvalid failures and enriches error_message.
src/openarmature/graph/events.py	Updates LlmRetryAttemptEvent documentation to describe the structured-output failure exception case.
docs/concepts/observability.md	Documents the structured-output failure exception for response-side surface rendering on error spans.
conformance.toml	Updates proposal 0082 note to reflect PR B’s OTel rendering + metric coverage.
CHANGELOG.md	Documents the new OTel error-span surface behavior and the added exception event on failed LLM attempts.