Function start_runtime_monitor 

pub fn start_runtime_monitor(
    runtime: &'static str,
    handle: &Handle,
    metrics: Arc<RuntimeMonitorMetrics>,
)

Starts background monitoring of the tokio runtime identified by handle, labelling all metrics with runtime.

Both the counter sampler and the scheduler-lag heartbeat run on the monitored runtime so that the heartbeat observes that runtime’s scheduling latency directly.