cubicweb #2179735 make it possible to retrieve some memory stats for an instance from pyro [validation pending]

Have a method similar to Repository.stats() that returns ths kind of information available on the GCView (/siteinfo#memory)

prioritynormal
typebug
done in3.14.4
load0.250
load left0.000
closed by#e35d4d4f7eb3 [repository] add a ``gc_stats`` method and 2 more cache size for ``stats`` (closes #2179735)