The address attribute interface was intended to provide a mechanism to
pass attributes separately from channel args, for values that do not
affect subchannel behavior and therefore do not need to be present in
the subchannel key, which does include channel args. However, the
mechanism as currently designed is fairly clunky and is probably not the
direction we will want to go in the long term.
Eventually, we will want some mechanism for registering channel args,
which would provide a cleaner way to indicate that a given channel arg
should not be used in the subchannel key, so that we don't need a
completely different mechanism. For now, this PR is just doing an
interim step, which is to establish a special channel arg key prefix to
indicate that an arg is not needed in the subchannel key.
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