anchor.diff:                          "temperature"
                       ]
                   },
  -                "callback": "http://localhost:1234",               
  +                "callback": "http://weather.example.com/ngsi",
  +                "metadata": {
  +                    "providingService": {
  +                       "value": "weather.example.com",
  +                       "type": null
  +                     },
  +                    "providingAuthority": {
  +                       "value": "AEMET - Spain",
  +                       "type": null
  +                     }
  +                },
                   "duration": "PT1M"
               }
           ]
  
positive.code:                 ]
              },
              "callback":  "http://localhost:1234",
              "duration": "PT1M"
          }
  
  
negative.code: 
  A context registration is represented by a JSON object with the following fields:
  
  + `id`       : Unique identifier assigned to the registration. Automatically generated at creation time.
  + `subject`  : It is an object that describes the subject of the registration.
  + `callback` : URL which denotes the end point of the registered service. It is optional, as it can be
  undefined under certain conditions, for instance, transient unavailability of the provider.
  + `metadata` : An optional JSON object which allows to associate extra metadata properties to the registration.
  It must follow the same representation conventions as context attribute metadata (See JSON Attribute Representation). 
  + `duration` : Duration of the registration in ISO8601 format. Default duration is infinite.
  
  A `subject` contains the following subfields:
  
  