[feature] Add cost metrics

From a quick look it seems like the metrics collected by the cost API are not included in the fastly exporter:
```
$ curl http://localhost:8080/metrics | grep invoices
<no result>
```

Would you be open to add this? We would be especially intereted in `/billing/v3/invoices/month-to-date` 😊 

https://www.fastly.com/documentation/reference/api/account/invoices/