Make interceptor Cluster bound instead of namespace bound

[nilesh93]

Interceptor should be able to run in an admin namespace (eg: keda-http) and should be able to forward traffic to multiple services in multiple namespaces

Use-Case

In a scenario where, there are multiple services in multiple namespaces in a given cluster, sometimes it could be costly to add a keda http interceptor per namespace. Rather, just like how ingress controller works, it would be best if the interceptor can reside in an admin namespace and forward traffic to multiple services in multiple namespaces.

[tomkerkhove]

This will be done through #183 where it will become multi-tenant.

[arschles]

@tomkerkhove actually, the interceptor after #183 (associated PR is #206) only works in the same namespace it's running in. that's because it forwards requests to http://<app_service>:<app_port>.

@nilesh93 we can make fairly simple changes to interceptor, scaler and HTTP Addon operator to respect a WATCH_NAMESPACE configuration parameter (like what the KEDA operator already respects) so that components are loosely coupled from their own namespace, but still single tenant.

the result would be that interceptor, scaler, addon operator and keda operator would still only work with one namespace at a time, but you could run many of them all in the same admin namespace, and configure each with a different target namespace (the same would go for scaler, addon operator, and keda operator). would that solution be workable for you?

[arschles]

note that this interacts with #241. if the operator were to create new interceptors and scalers on submission of a new HTTPScalingComponents CRD, then it would be responsible for setting up the WATCH_NAMESPACE environment variable. we'll need to consider that when finalizing a design here (and vice versa). ideally, I'd like a single solution to solve both concerns at once

[stale]

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