Apparently, host field is filled with the value of the host the beat is running on if the urls are NOT ips. Otherwise it is filled with the ip.
Example:
- name: first
type: http
urls:
- https://123.45.6.7:443
- name: second
type: http
urls:
- https://www.myurl.com:443
Result:
First:
...
"host": "heartbeat.machine"
Second:
...
"host": "www.myurl.com"
