Function init 

pub fn init(format: LogFormat, default_filter: &str) -> bool

Initialise the global tracing subscriber.

The filter comes from RUST_LOG when set, else default_filter (e.g. "info,etl_core=debug"). Idempotent: returns true if this call installed the subscriber, false if one (ours or foreign) was already installed — never panics, so libraries and tests can call it freely.