Metering is watching usage of the system, usually for the purposes of Billing.
Think of your power meter. You can go outside and watch the dial spin and confirm your monthly bill jives with what the meter is reporting.
The important aspects of metering:
These events/measurements are critical. We cannot risk dropping an event.
We need to ensure these events are consistent between releases.
We have no idea how people are going to want to use these events, but we can safely assume there will be a lots of other groups interested in them.
These events may not be nearly as frequent as the instrumentation messages, but they will be a lot larger since the entire context of the message needs to be included (which instance, which image, which user, etc)