NGSI patching enhancement to support metadata

[fgalan]

Is your feature request related to a problem / use case? Please describe.

Currently we can add and modify attribute values in custom notification using ngsi field (NGSI patching), but not metadata.

Describe the solution you'd like

The solution would be to enhance the current NGSI patching feature to allow also add/modify metadata in attributes.

Describe alternatives you've considered

None

Describe why you need this feature

• To approach a new use case (not yet clear which use case make take advantage of this, but it could be useful)