Attribute's metadata update fails when no value is given for such an attribute

[frbattid]

Orion warns that updating an attribute's metadata when no value is given for such attribute is an offending operation:

$ cat updateContext.sh
(curl localhost:1026/v1/updateContext -s -S --header 'Content-Type: application/json' --header 'Accept: application/json' -d @- | python -mjson.tool) <<EOF
{
    "contextElements": [
        {
            "type": "Room",
            "isPattern": "false",
            "id": "Room1",
            "attributes": [
            {
                "name": "temperature",
                "type": "float",
                "metadatas": [
                {
                        "name": "max",
                        "type": "double",
                        "value": "50"
                }
                ]
            },
            {
                "name": "pressure",
                "type": "integer",
                "value": "763"
            }
            ]
        }
    ],
    "updateAction": "UPDATE"
}
EOF

$ ./updateContext.sh 
{
    "contextResponses": [
        {
            "contextElement": {
                "attributes": [
                    {
                        "name": "temperature", 
                        "type": "float", 
                        "value": ""
                    }
                ], 
                "id": "Room1", 
                "isPattern": "false", 
                "type": "Room"
            }, 
            "statusCode": {
                "code": "472", 
                "details": "action: UPDATE - entity: [Room1, Room, false] - offending attribute: temperature - empty attribute not allowed in APPEND or UPDATE", 
                "reasonPhrase": "request parameter is invalid/not allowed"
            }
        }
    ]
}

Why is it necessary to provide a value for the attribute? I mean, I'm updating the attribute's metadata, not the attribute itself. This kind of behavior avoids this data lifecycle I was implementing through Cygnus:

1. An attribute's change is notified by Orion to Cygnus.
2. Cygnus uses the notified value in order to update certain statistics about the attribute such as maximum and minimum value, or average value.
3. Cygnus updates the attribute's metadata by pushing the calculated max, min, average, etc, but not updating the attribute itself.

Effort: 2 man day

[fgalan]

Addressed in PR #1009