Finding out which hosts are sending logs

I want to find out all hosts (Azure VMs) which are sending log data to GrayLog/Elastic. Is it possible to get the ip address of the host. I used the query below and its giving me the hostname. Unfortunatelly this is overwritten by some admins and does not show me the VM name in Azure.
Is it possible to extract the IP address of the VM sending logs?

Graylog: 2.1.3
Elasticsearch: 2.4.4

My query:
curl --silent -XGET 'XX.XXX.XX.XX:9200/_all/_search?pretty=true' -H 'Content-Type: application/json' -d '
"size": "0",
"aggregations": {
"uniq_hosts" : {
"terms": {"field": "source", "size": 1000}