add metadata to ngsild payload measure + doc

docs/usermanual.md

@@ -243,7 +243,10 @@ Example of these `ngsild` payloads are the following ones:
 }
 ```
 
-Note that array of entities are handled as a multiple measure, each entity is a measure.
+Some additional considerations to take into account:
+
+-   In the case of array of entities, they are handled as a multiple measure, i.e. each entity is a measure.
+-   In the case of NGSI-LD, fields different from `type` or `value` (e.g. `observedAt` in the examples above) are include as NGSI-v2 metadata      in the entity corresponding to the measure at Context Broker.
 
 ##### SOAP-XML Measure reporting
 

lib/commonBindings.js

@@ -133,6 +133,15 @@ function extractAttributes(device, current, payloadType) {
                                     ent.value = entity[k].object;
                                 }
                             }
+                            // Add other stuff as metadata
+                            for (let key in entity[k]) {
+                                if (!['type', 'value', 'object'].includes(key.toLowerCase())) {
+                                    if (!ent.metadata) {
+                                        ent.metadata = {};
+                                    }
+                                    ent.metadata[key] = entity[k][key];
+                                }
+                            }
                         }
                         valuesEntity.push(ent);
                     }

test/unit/ngsiv2/contextRequests/ngsildPayloadMeasure.json

@@ -3,7 +3,10 @@
     "type": "AnMQTTDevice",
     "status": {
         "type": "Property",
-        "value": "free"
+        "value": "free",
+        "metadata": {
+            "observedAt":"2018-09-21T12:00:00Z"
+        }
     },
     "category": {
         "type": "Property",

test/unit/ngsiv2/contextRequests/ngsildPayloadMeasure2.json

@@ -3,7 +3,10 @@
     "type": "AnMQTTDevice",
     "status": {
         "type": "Property",
-        "value": "free"
+        "value": "free",
+        "metadata": {
+            "observedAt":"2012-09-21T12:00:00Z"
+        }
     },
     "category": {
         "type": "Property",