Add method to update Monitoring::Hub::Entity from JSON data

LHCb needs to be able to dump counters (or any monitorable thing) to JSON (similar to what happens with !1362 (merged)) and to update counters in memory from JSON data (an extension of what !1253 (merged) and !1278 (merged) introduced).

Before actually adding the feature I had to fix the fromJSON methods of the counters, which were never used, so never checked by the compiler (I introduced unit tests for the feature).