After looking at #4245 I started wondering if we could set up user defined headers which would tickle our new release assert (and formerly OOM). At least in unit test (coming soon) we can.

While hopefully there's sanity checking of user defined config before it hits Envoy, I also think we need some sane limits on user defined headers to reduce the likelihood of malicious users configuring themselves an Envoy Query of Death.

By default I think I'd like a smallish limit on number of user defined headers, length of individual header lines and maybe another one on total header size. I'd lean towards starting restrictive and allowing configuring permissiveness as folks need it but this could break existing behavior so needs some discussion.

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