Function collect_targets 

pub fn collect_targets(
    graphs: &[Arc<CompiledGraph>],
    id: &str,
) -> Vec<CacheTarget>

Every distinct backend holding entries for id, across all compiled graphs.

Deduplicated by (backend_label, store): every policy: local half in the process shares one LocalResponseCache, and two policies can point at the same redis store. Purging a shared backend once per half would repeat the work and report a count that means nothing.