[Spike] Investigate options to eliminate hard coded metricids in getInstancesByProduct API sorting key

Currently when calling the getInstancesByProduct() api one of the options is to sort the results by a measurement. Unfortunately today, the specification of the sortable items is an enum in the API spec [1]. This is a problem because the enumeration has to be updated whenever a new sortable measurement type is added. The possible values for metric id for a given product are coming from configuration. This puts us in a weird place where half the values cleanly map to an enumeration and half of them are custom per product. 

This card is to investigate what the options are and present a proposal so that we do not have to update an enumeration in an API spec every time we add a new metricid for a new product. 

[1]https://github.com/RedHatInsights/rhsm-subscriptions/blob/d12cb4833954dda394fdee5dfe8a5e11da0f5dcc/api/rhsm-subscriptions-api-spec.yaml#L848