Function build_zipkin 

fn build_zipkin(
    span: &SpanContext,
    ctx: &Context,
    service_name: &str,
    server_addr: Option<&str>,
) -> Value

Builds the Zipkin v2 JSON span array for a finished span (pure; no I/O).

timestamp and duration are microseconds (Zipkin’s unit); parentId is omitted for a root span.