cubicweb #2951068 use the call_services API to retrieve repo gc_stats [validation pending]

We currently add a new method on repo that anyone can call. Call service allows to reach the same result with: - restrict access to manager - do not bloat the Repository class

done in3.19.0
load left0.000
closed by#63f3d25bab14 [repo] move repo.gc_stats to Service API (closes #2951068)
patch[repo] move repo gc_stats to Service [applied]