Per-filter child spans

[shaneutt]

Create child spans for each filter execution per phase with filter name, duration, and result.

Requirements

• Child span per filter per phase (request_headers, request_body, response_headers, response_body)
• Span name: filter:{filter_name}:{phase}
• Attributes: filter name, phase, result (continue/reject/error)
• Duration covers filter execution time
• Nested correctly under root request span

Implementation Notes

• Instrumentation in filter pipeline executor, not in individual filters
• Minimal overhead when tracing is disabled

Parent epic: #10