Macro spawn_metrics 

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

Prepare metrics for a spawned task.

This macro returns a tuple (Label, Gauge):

• Label: A label representing the task’s metrics.
• Gauge: A gauge tracking the number of running tasks with the given label.