Unable to create UDM event

New to Google secops.

I have an application that emits network connection/flow events, which I am trying to map to udm event.

Referred,

1. https://docs.cloud.google.com/chronicle/docs/reference/udm-field-list#Noun
2. https://docs.cloud.google.com/chronicle/docs/reference/udm-field-list#metadataeventtype
3. https://docs.cloud.google.com/chronicle/docs/unified-data-model/udm-usage#required_and_optional_entity_fields

And ended up with,

        {
        "metadata": {
            "eventTimestamp": "current_time",
            "id": "event_id",
            "eventType": "NETWORK_CONNECTION",
            "vendorName": "company",
            "productEventType": "Risky Services Insight",
            "description": "1024 bytes max desc",
            "productLogId": "8b2373b6-afa5-55a7-93da-cced77fd5f7f",
            "urlBackToProduct": "some url",
        },

        "principal": {
            "port": 7000,
            "application": "virtual_machine",
            "assetId": "some asset",
            "hostname": "pinguino-01",
        },
        "target": {
            "hostname": "pinguino-01",
        },
        "network": {
            "applicationProtocol": "AFP",
            "ipProtocol": "TCP",
            "receivedBytes": 46602,
            "sentBytes":  85083,
          }
    }

I am calling ingest_udm method in a python script, this method belongs to google sdk for secops.

It fails with, 

Error ingesting event: Failed to ingest UDM events: {
  "error": {
    "code": 400,
    "message": "ImportEvents failed",
    "status": "INVALID_ARGUMENT"
  }
}