It all depends on the event's type. E.g compute.instance.* events:

- event_type: compute.instance.*
  traits: &instance_traits
    tenant_id:
      fields: payload.tenant_id
    user_id:
      fields: payload.user_id
    instance_id:
      fields: payload.instance_id
    host:
      fields: publisher_id.`split(., 1, 1)`
    service:
      fields: publisher_id.`split(., 0, -1)`
    memory_mb:
      type: int
      fields: payload.memory_mb
    disk_gb:
      type: int
      fields: payload.disk_gb
    root_gb:
      type: int
      fields: payload.root_gb
    ephemeral_gb:
      type: int
      fields: payload.ephemeral_gb
    vcpus:
      type: int
      fields: payload.vcpus
    instance_type_id:
      type: int
      fields: payload.instance_type_id

As U can see - there is a bunch of useful info.

For other traits available in Juno release check /etc/ceilometer/event_definitions.yaml

And definitely - http://docs.openstack.org/developer/c...