Investigate LB from wasm in Envoy, others?

[alexsnaps]

Envoy lets you slightly control LB based on a header's value. If that header is set by the wasm-shim can we use it as a way to influence the load-balancing across upstreams? Is that purely based on constant hashing or can we specifically control to which upstream the request gets routed? Possibly using some other mechanism than the header?

What do other proxies out there do? Whether lua/wasm or whatever?