TCP connection lifecycle spans

[shaneutt]

Spans for TCP proxy connection lifecycle including byte counts and duration.

Requirements

• Span per TCP connection lifecycle
• Attributes: client address, upstream address, bytes sent, bytes received
• Attributes: connection duration, TLS handshake duration (if applicable)
• Span events for connection open, close, error
• Support for TcpFilter pipeline tracing

Implementation Notes

• TCP spans are independent root spans (not children of HTTP spans)
• Byte counts updated at connection close

Parent epic: #10