Function start_probe_timer_if 

pub fn start_probe_timer_if<B: Backend>(
    enabled: bool,
    ctx: &mut B::Context,
) -> Option<B::Timer>

Start a timer iff enabled is true — None is the disabled state. Pair with finish_probe_timer at the end of the scope. The env/config gate is intentionally resolved by the caller so hot probes do not read process env while a token/layer loop is running.