Function propagatable_attrs 

pub fn propagatable_attrs() -> Attrs

Return a complete, merged snapshot of the effective configuration (only keys marked with @meta(CONFIG = ...) and propagate: true).

Resolution per key:

1. First explicit value found in layers (TestOverride → Env → Runtime → File → ClientOverride).
2. Otherwise, the key’s default (if any).

Notes:

• This materializes defaults into the returned Attrs for all CONFIG-marked keys with propagate: true.
• Keys without CONFIG meta are excluded.
• Keys with propagate: false are excluded.

Use this when sending config to child processes via BootstrapProcManager. Process-local configs (like TLS cert paths) should have propagate: false and will not be included.