What is a metering frequency and why would an airport have it?

Use of a metering controller and frequency at an airport is one strategy used to prevent congestion on ramps and taxiways. When ready for push back from the gate, pilots call metering. The metering controller will typically either give them permission to push with a ground frequency to call when ready for taxi, or they will tell them to monitor a particular ground frequency for permission to push. They may even tell the pilots to stay on the gate and call back after a specified delay. The use of metering allows the airport controllers to manage ramp and taxi space by sequencing aircraft push and taxi.

Metering control can also help manage potential congestion due to ground delays for situations such as poor weather, runway closures, deicing, flow times into particular destination airports or any other situation that increases taxi time or slows down departures.

Some airports uses different strategies such as discrete ramp areas and frequencies to coordinate push back and taxi flow or internal coordination between controllers.

Have you ever driven into a freeway on-ramp and ran into a traffic light that tells drivers when they can proceed? It's usually turned on during rush hour to prevent jams and accidents. Same thing. It's a flow control method. That pilot was probably unhappy at how long he/she had to wait to be handed over to Ground Control so they could pushback/taxi after contacting the controller on the metering frequency.