support multiple HostLbPolicyData entries per host

[jukie]

Based on feedback in #43784.
This converts HostLbPolicyData from single-slot to a vector so multiple LB policies (e.g. the new policy in #43784 plus CSWRR as a child) can each attach per-host state and receive ORCA reports independently.

[repokitteh-read-only]

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🐱

Caused by: #43995 was opened by jukie.

see: more, trace.

[wbpcode]

LGTM. Thanks for this great contribution. And defer this to @paul-r-gall for another check. :)

[paul-r-gall]

@adisuissa lmk what you think about this API. I think it solves the problem, but I still with that "orca math" were not even part of the router filter at all.

[paul-r-gall]

Actually, the default return value here is harmful; imagine if someone had their own implementation of this thing that relied on orca reports. Rather than ceasing to compile, their implementation would start to misbehave. In particular, I recommend that you either make this a pure method or have the default value be true.

[adisuissa]

I still think the bigger problem is that instead of having the 'HostLbPolicyData' in the host it should be part of the LB-policy implementation. But modifying that is probably a big change.

So the question is whether the suggested solution causes some strange side-effects or greatly complicates its maintenance. I don't have sufficient context to answer this at the moment.

[jukie]

I'd be willing to work on moving HostLbPolicyData out from host-level but it does seem like a non-trivial change. Would it be reasonable to take that on in a follow-up and use this as the first step?

[jukie]

@paul-r-gall updated with your feedback