anchor.diff:                  ]
               },
               "callback":  "http://localhost:1234",
  +            "metadata" : {
  +                "provider": {
  +                   "value": "example.com"
  +                }
  +            },
               "duration": "PT1M"
           }
   
  
positive.code:                         "temperature"
                      ]
                  },
                  "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:
  
  