Macro spawn_metrics 

macro_rules! spawn_metrics {
    ($ctx:ident) => { ... };
    ($label:expr, @make $ctx:ident) => { ... };
}

Prepare metrics for a spawned task.

Returns a (Label, MetricHandle) pair for tracking spawned tasks.

The Label identifies the task in the metrics registry and the MetricHandle immediately increments the tasks_running gauge for that label. Call MetricHandle::finish once the task completes to decrement the gauge.