Draining `inboundonly` and `graceful` do not work together

[howardjohn]

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Title: Draining inboundonly and graceful do not work together

Description:

when draining listeners with inboundonly&graceful&skip_exit, the inboundonly attribute is ignored.

You can see here it is never used at all: https://github.com/envoyproxy/envoy/blob/277722e94187045bfe7da6d13d2835b5c5b14168/source/server/admin/listeners_handler.cc#L31-L43

Repro steps: curl httpbin.org/get -v and look for connection: close header. Note this goes through an outbound listener.

• Before: no close.
• curl -X POST localhost:15000/'drain_listeners?inboundonly: no close header set
• curl -X POST localhost:15000/'drain_listeners?inboundonly&graceful&skip_exit: close header is set unexpectedly

[keithmattix]

/assign @keithmattix

[keithmattix]

Update: the drain manager doesn't actually support uni-directional draining. This would be a new feature, but I think it's reasonable and somewhat simple to implement. I can potentially get a PR out in the next week or so

[keithmattix]

/cc @jmarantz what're your thoughts here? The draining semantics that I see in the code don't have any existing hooks for limiting the draining to only inbound listneners