The key to use for filtering data. For example, QUEUE, ROUTING_PROFILE, AGENT, CHANNEL, AGENT_HIERARCHY_LEVEL_ONE, AGENT_HIERARCHY_LEVEL_TWO, AGENT_HIERARCHY_LEVEL_THREE, AGENT_HIERARCHY_LEVEL_FOUR, AGENT_HIERARCHY_LEVEL_FIVE. There must be at least 1 key and a maximum 5 keys.