How to use `jq` to obtain the keys

My json looks like this :

{"20160522201409-jobsv1-1": {"vmStateDisplayName": "Ready","servers": {"20160522201409 jobs_v1 1": {"serverStateDisplayName": "Ready","creationDate": "2016-05-22T20:14:22.000+0000","state": "READY","provisionStatus": "PENDING","serverRole": "ROLE","serverType": "SERVER","serverName": "20160522201409 jobs_v1 1","serverId": 2902      }    },"isAdminNode": true,"creationDate": "2016-05-22T20:14:23.000+0000","totalStorage": 15360,"shapeId": "ot1","state": "READY","vmId": 4353,"hostName": "20160522201409-jobsv1-1","label": "20160522201409 jobs_v1 ADMIN_SERVER 1","ipAddress": "10.252.159.39","publicIpAddress": "10.252.159.39","usageType": "ADMIN_SERVER","role": "ADMIN_SERVER","componentType": "jobs_v1"  }}

My key keeps changing from time to time. So for example 20160522201409-jobsv1-1 may be something else tomorrow. Also I may more than one such entry in the json payload.

I want to echo $KEYS and I am trying to do it using jq.

Things I have tried : | jq .KEYS is the command i use frequently.

Is there a jq command to display all the primary keys in the json?

I only care about the hostname field. And I would like to extract that out. I know how to do it using grep but it is NOT a clean approach.