Failing metadata plugin results in UNAVAILABLE

[jtattermusch]

Context: grpc/grpc#13322

Needs to be merged before grpc/grpc#13363. The change is a little awkward, but since grpc-node now lives in a different repository, we don't have merge atomicity anymore and we can't really come up anything better without breaking the test temporarily.

[jtattermusch]

CC @jboeuf

[murgatroid99]

We're just going to have to accept that that test is going to break sometimes. I don't think we need to make this kind of transitional change. We should just make the actual change with the next submodule update.

[jtattermusch]

I'm fine with breaking the tests for duration of ~minutes (basically having all the PRs ready to merge), but knowingly introducing a breakage for longer time is wrong. What is the right ordering of PRs to minimize the impact?
Is this correct:

1. merge this PR (after modifying it to only accept UNAVAILABLE status) - that will break the grpc/grpc tests temporarily
2. merge Revert "Revert "Switching from UNAUTHENTICATED to UNAVAILABLE for auth metadata failure"" grpc#13363 shortly after 1) which will fix the tests in both grpc/grpc and grpc-node.

[murgatroid99]

I have opened #90, which is my preferred solution. You should merge your PR on grpc/grpc, then I will update that one and merge it. The grpc/grpc tests will be broken temporarily.

[nicolasnoble]

I would side with @murgatroid99 here that we shouldn't try making commits that handles two things at once. Unless the change is too big, preparing the change such as #90 and committing grpc/grpc#13363 in almost lockstep is the better way to handle this.

[jtattermusch]

Closing in favor of #90.