Macro handler_debug_span 

macro_rules! handler_debug_span {
    (target: $target:expr, $msg:expr) => { ... };
    (target: $target:expr, $msg:expr, $($extra_fields:tt)*) => { ... };
}

A macro that lets attach handle() functions to the tracing context that generated the actix message being processed. Creates a DEBUG-level span with the handler type name and the message type name as attributes.