Span events for rejection, timeout, retry, error

[shaneutt]

Record span events at key decision points in request processing.

Requirements

• Event on request rejection (with reason, rejecting filter)
• Event on upstream timeout (with timeout duration, upstream address)
• Event on retry attempt (with attempt number, reason)
• Event on error (with error type, message)
• Event on circuit breaker trip

Implementation Notes

• Events are lightweight annotations on existing spans
• Include relevant context as event attributes

Parent epic: #10