Make field to use for TenantUsage configurable
Per https://whmcs.community/topic/297347-usage-metrics-not-updating/ the identifier for usage metrics must be stored in the domain field. This is very rigid and should be configurable so that services that don't use a domain aren't forced to. Setting the domain field to a numbered ID is confusing to users.
Specifically, I'd like to be able to use the service ID as the key, but ideally I could use any field.