subset lb: support customized response code when using NO_FALLBACK
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area/load balancing
design proposal
help wanted
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Description
We have a use case for the subset LB in which we'd like to return a 404 when there is no metadata/subset match. Right now, a 503 (no healthy hosts) response is the only possible response:
https://github.com/envoyproxy/envoy/blob/master/api/envoy/config/cluster/v3/cluster.proto#L206
We could make this knob something specific to the subset LB; or we could make it something generic for all LBs similar to `ClusterNotFoundResponseCode` (e.g.: `EndpointNotFoundResponseCode`):
https://github.com/envoyproxy/envoy/blob/master/api/envoy/config/route/v3/route_components.proto#L794
Thoughts?
cc: @zuercher @fishcakez @janephilipps @mattklein123
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