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: 
  A context registration is represented by a JSON object with the following fields:
  
  + `id`: Unique identifier assigned to the registration. Automatically created at creation time.
  + `subject` : It s an object that describes the subject of the registration.
  + `callback` : URL pointing to the service which is registered. In the case of a Context Provider
  corresponds to the URL of the provider service.
  + `duration`: Duration of the registration in ISO8601 format. Default duration is infinite.
  
  A `subject` contains the following subfields:
  
  
negative.code:                 ]
              },
              "callback":  "http://localhost:1234",
              "metadata" : {
                  "provider": {
                     "value": "example.com"
                  }
              },
              "duration": "PT1M"
          }
  
  