Add microbenchmark for LongKeyedBucketOrds (backport of #58608)

[nik9000]

I've always been confused by the strange behavior that I saw when
working on #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 #56487