Function build_sw8 

fn build_sw8(
    span: &SpanContext,
    service: &str,
    instance: &str,
    endpoint: &str,
) -> String

Builds a downstream sw8 header value for span (8 hyphen-separated fields, matching the SkyWalking cross-process propagation header): sample-traceId-parentSegmentId-parentSpanId-service-instance-endpoint-peer. All fields except the sample flag and the plain-integer parent span id are base64-encoded.