Emit model usage events

[amorton]

Now we have #2008 add feature flags to configure emitting model usage events in headers.

We want billing events for the egress bytes, ingress bytes, and token usage.

Use feature flags to enable / disable at config time and at request time.

As part of this, also add metrics for ModelUsage

[amorton]

this is the ticket for emitting the headers for the usage