Function get 

pub fn get<T: AttrValue + Copy>(key: Key<T>) -> T

Get a key from the global configuration (Copy types).

Resolution order: TestOverride -> Env -> Runtime -> File -> ClientOverride -> Default. Panics if the key has no default and is not set in any layer.

This function reads from a pre-materialized snapshot for lock-free performance. The snapshot is updated atomically whenever layers change.