Root span per request with standard attributes

[shaneutt]

Create a root span for each request with standard HTTP semantic convention attributes.

Requirements

• Root span created at request ingress
• Attributes: HTTP method, path, status code, client IP, request ID
• Attributes: upstream cluster name, upstream address
• Span name: {method} {path} (e.g. GET /api/v1/models)
• Span kind: SERVER
• Duration covers full request lifecycle
• Request ID must be a recorded span field (not just
  an attribute) so it propagates into every tracing
  log line emitted within the span tree — filter
  execution, upstream connect, error events, etc.
  This enables grep-by-request-ID across all log
  output, not just access logs.

Implementation Notes

• OpenTelemetry HTTP semantic conventions for attribute names
• Request ID from RequestIdFilter output

Parent epic: #10