fix skip label check

[reyreaud-l]

Memberlist allows having label and to check them on receiving a message to ensure rings don't collide unexpectedly. The option to skip that checks currently doesn't work and will fail the message unless the labels are exactly the same.

This PR change that behaviour to ensure that the option to skip the label check does that.

I tried adding test to showcase that the behaviour is now working as expected. Let me know if you any changes would be necessary. This is my first time contributing to that repository, let me know if I missed any necessary steps <3

[hashicorp-cla-app]

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[rboyer]

This is working as intended.

SkipInboundLabelCheck is about loosening the requirement that inbound traffic be labeled at all, not that it be labeled correctly. This is expected to be set when the label has been received and removed by a proxy of sorts, before arriving at the memberlist where SkipInboundLabelCheck is set to true. Internally that latter memberlist is configured to know its label and must ensure that the protocol was not violated and accidentally two labels were sent.

[reyreaud-l]

Hey @rboyer thanks for the feedback. If that's not the use for SkipInboundLabelCheck, how would you recommend doing an HA rollout on a memberlist ring where one is introducing a label ?

For example you have a ring with 2 nodes and no label, you want to add a label to the ring to ensure that this ring will not collide with other rings unexpectedly. You need to do that while keeping at least one node online.

With my change (and the behaviour that I assumed of the flag). You could do

1. Rollout to node one by one with SkipInboundLabelCheck to true
2. Rollout with adding the label one by one
3. Rollout with removing SkipInboundLabelCheck

But that's not working with the current memberlist impl. Would you have any pointers to do it ? Would you be open to adding a new flag that is doing what I'm describing (ie skip all label checks) ?