This feature would allow API owners to give access to API consumers their daily/monthly call consumption via response headers.

The Response Headers would contain the following:
headerLimit: The limit being applied to the service
headerRemaining: How many requests remain before the limit is reached.
headerReset: The number of seconds until the limit is reset.

This functionality is part of APIMan and is described by the following:
http://www.apiman.io/latest/user-guide.html#_rate_limiting_policy