Provide a way to attach dynamic data to Host object

[efimki]

Title: Provide a way to attach dynamic data to Host object

Description:
I'm prototyping a new LB policy that will calculate host weights for WRR locally, based on custom headers attached to the backend responses.

The LB policy needs to keep dynamic internal data on per host basis, similar to HostDescription::metadata()

I think FilterState serves similar purpose, but it doesn't attach to a Host.

Are there better alternatives, or would it be possible to attach dynamic data (similar to FilterState) to a Host or HostDescription?

[tonya11en]

Are you trying to do something where the upstream hosts report their load via response headers and scale weights based on this? If so, it's the same as the ongoing conversation in #20907.

Otherwise, if you're implementing a custom LB extension are you not able to modify the Host objects in the implementation or keep some mapping of host->derived weight?

[efimki]

Yes, I'm trying to implement a ClientSideWeightedRoundRobin LB Policy that would use LoadMetricStats collected by an upstream filter that extracts them from response headers in the format of OrcaLoadReport.

[efimki]

Are you suggesting to extend Host object specifically in custom LB extension or something else?
Keeping some mapping of host->derived is certainly an option, but seems a bit clunky.

[adisuissa]

cc @wbpcode who may be interested in this new LB policy