GET /attributes, with details=true, and registrations

[kzangeli]

For now, only local entities are considered for GET /attributes?details=true.
This is how the response payload body looks when details is turned on:

[
  {
    "id": "https://uri.etsi.org/ngsi-ld/default-context/P1",
    "type": "Attribute",
    "typeNames": [ "T", ... ]
    "attributeCount": 2,
    "attributeName": "P1",
    "attributeTypes": [ "Property", ... ],
  },
  {
    ...
  },
  ...
]

Easy enough to extract all that data for attributes of local entities, but, what about registrations?

In the case of iot agents, I guess the normal way is that an entity is registered with its entity id and type, and the list of attributes.
With that I could easily extract info for typeNames, and attributeCount (I'd increment it by one). The necessary info for attributeTypes is not so easy though; I'd have to query the iot agent to retrieve the type of the attributes.
Also, what if this registration is done in more than one broker? Could be counted more than once.
I'd need the registration to be "local" to make sure it's counted for just once.

Then we have the problem with more open registrations, like just an entity type.
For open registrations, I'd need to query the provider about the list of attributes.
It all gets extremely complicated and a clear definition on forwarding (global and local registrations) and federation will be needed before I can make a decision on how to fully implement `GET /attributes?details=true'.

So, for now, Orion-LD only considers local entities for `GET /attributes?details=true'.