Make clear that proxy metadata must be known to the client

[chris-wood]

Split out from #96, since the shadowban bit needs more consideration in relation to target->proxy signals like rate limits.

Closes #100.

cc @ekr

[martinthomson]

Do we need to be clearer about the client knowing the type of information that is added, though it might be ignorant of the specific value the proxy chooses?

[chris-wood]

Do we need to be clearer about the client knowing the type of information that is added, though it might be ignorant of the specific value the proxy chooses?

We could list examples, but it would be a non-exhaustive list that might end up harming more than helping, so I'm inclined to omit that for now.

[martinthomson]

I have some small suggestions here. I think that we need to sharpen the distinction between knowledge of the type of information that might be added and the specific value that is added.

A small nit about the halving thing made me think a little...

[martinthomson]

WFM