mediator: built-in middlewares (TracingMiddleware, LoggingMiddleware, TimeoutMiddleware)

[pierrick-fonquerne]

Ship three production-ready middlewares so users get observability out of the box without writing boilerplate.

Scope

• TracingMiddleware: opens an OpenTelemetry span per dispatch keyed on command type, MessageId and CorrelationId.
• LoggingMiddleware: emits a tracing::info! event before and after each dispatch with elapsed time.
• TimeoutMiddleware: cancels the dispatch after a configurable Duration, returns HexeractError::Timeout.

Acceptance criteria

• TracingMiddleware records message_id and correlation_id as span attributes.
• LoggingMiddleware output includes the command type name and elapsed milliseconds.
• TimeoutMiddleware returns HexeractError::Timeout when the handler exceeds the deadline.
• All three middlewares implement Clone and are usable without Arc wrapping.