AJ Heller

I see the mutex re-acquisition problem https://source.cloud.google.com/results/invocations/419395f4-3a67-4bd1-9c73-f3fe2a43c1e5/targets/%2F%2Ftest%2Fcore%2Fend2end:connectivity_test@experiment%3Dpromise_based_client_call/log

@youyuanwu I have a fix in the linked PR. Thanks for helping us find some bugs!

@youyuanwu Your change is merged, thanks for adding this feature!

> Hey, did this make it into main? Yes. https://github.com/grpc/grpc/commit/a58f83c947be52d4b622aa0d32435323b81a8e5e

Depends on #35648

No longer necessary, the passthru endpoint was removed in https://github.com/grpc/grpc/pull/35986