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

Currently when deriving EncodeLabelSet and flattening a struct the flattened struct must appear last, and there must be only one flattened struct, from the test:

#[test]
fn flatten() {
    #[derive(EncodeLabelSet, Hash, Clone, Eq, PartialEq, Debug)]
    struct CommonLabels {
        a: u64,
        b: u64,
    }
    #[derive(EncodeLabelSet, Hash, Clone, Eq, PartialEq, Debug)]
    struct Labels {
        unique: u64,
        #[prometheus(flatten)]
        common: CommonLabels,
    }

    // …

If you place common before unique in struct Labels like this:

#[derive(EncodeLabelSet, Hash, Clone, Eq, PartialEq, Debug)]
struct Labels {
    #[prometheus(flatten)]
    common: CommonLabels,
    unique: u64,
}

Compilation fails:

error[E0382]: borrow of moved value: `encoder`
   --> derive-encode/tests/lib.rs:147:14
    |
147 |     #[derive(EncodeLabelSet, Hash, Clone, Eq, PartialEq, Debug)]
    |              ^^^^^^^^^^^^^^
    |              |
    |              value borrowed here after move
    |              move occurs because `encoder` has type `LabelSetEncoder<'_>`, which does not implement the `Copy` trait
    |

This occurs because ownership of encoder is given to the flattened struct

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.

Start in derive-encode/src/lib.rs at the ownership transfer linked by the issue, then inspect the flatten test in derive-encode/tests/lib.rs. Add coverage for a flattened field before another field and for multiple flattened structs, and verify that these declarations compile without the encoder move error.