Proposal: ability to change rate limit descriptor key generated by generic_key action

[Pchelolo]

Rational

The generic_key rate limit action allows to set a descriptor value to a configured value, however the key is always "generic_key". This makes the ratelimit service config less expressive. Descriptor (generic_key: route_name_XXX) looks much less expressive then descriptor (route_name: XXX)

Proposal

Add optional descriptor_key string property to the generic_key action. If set, it would hold the descriptor key. If not, default to 'generic_key' key.

We would provide the implementation/testing, seeking comments/approval of the proposal.

[mattklein123]

SGTM!