I've always been confused by the strange behavior that I saw when
working on elastic#57304. Specifically, I saw switching from a bimorphic
invocation to a monomorphic invocation to give us a 7%-15% performance
bump. This felt *bonkers* to me. And, it also made me wonder whether
it'd be worth looking into doing it everywhere.

It turns out that, no, it isn't needed everywhere. This benchmark shows
that a bimorphic invocation like:
```
LongKeyedBucketOrds ords = new LongKeyedBucketOrds.ForSingle();
ords.add(0, 0); <------ this line
```

is 19% slower than a monomorphic invocation like:
```
LongKeyedBucketOrds.ForSingle ords = new LongKeyedBucketOrds.ForSingle();
ords.add(0, 0); <------ this line
```

But *only* when the reference is mutable. In the example above, if
`ords` is never changed then both perform the same. But if the `ords`
reference is assigned twice then we start to see the difference:
```
immutable bimorphic    avgt   10   6.468 ± 0.045  ns/op
immutable monomorphic  avgt   10   6.756 ± 0.026  ns/op
mutable   bimorphic    avgt   10   9.741 ± 0.073  ns/op
mutable   monomorphic  avgt   10   8.190 ± 0.016  ns/op
```

So the conclusion from all this is that we've done the right thing:
`auto_date_histogram` is the only aggregation in which `ords` isn't final
and it is the only aggregation that forces monomorphic invocations. All
other aggregations use an immutable bimorphic invocation. Which is fine.

Relates to elastic#56487