Allow flattening of a struct through derive(EncodeLabelSet) at any position about client_rust

[mxinden]

Thanks for debugging and the detailed issue.

I think this would be a breaking change to fix, but fixing it would allow fields to appear in any order, or allow flattening of multiple structs without nesting them all one inside the other in tail position.

Worth exploring passing &mut LabelSetEncoder. Not sure why I chose to require ownership.

Another alternative is to provide a better error message in the derive macro when the flattened property is not the last.

[olix0r]

Worth exploring passing &mut LabelSetEncoder.

My initial impression is that this would be a welcome API change. If we could impl<A: EncodeLabelSet, B: EncodeLabelSet> EncodeLabelSet for (A, B), that would generally make label composition more flexible.