Consistent prefix for system events and metrics

[mthenw]

Right now we use gateway. prefix in both system events and metrics. Especially for metrics we need to be more specific and gateway sounds like a bit too generic. My proposition is to change the prefix to eventgateway or event-gateway.

Another option is to use different prefix for system event and different for metrics. For metrics it's probably OK to prefix them with eventgateway.

For system events, as they are CloudEvents we could probably do something like com.serverless.eventgateway.

[austencollins]

Follow CloudEvents conventions whenever possible.

Also, a useful reference: https://stripe.com/docs/api#event_types