Upstream connection and exchange spans

[shaneutt]

Spans for upstream connection establishment and request/response exchange timing.

Requirements

• Child span for upstream connection establishment (connect duration)
• Child span for upstream request send and response receive
• Attributes: upstream address, port, TLS version (if applicable)
• Attributes: response status code, response body size
• Connection reuse noted as span attribute

Implementation Notes

• Spans nested under root request span
• Connect span covers DNS + TCP + TLS handshake

Parent epic: #10