Now that we're consistently using `cat_match` to filter which shards we
run on we can get this confusing case:
1. You have a search with, say, a range and a sub-agg.
2. That search has a query that `can_match` can recognize will match no
docs. On *any* shard.
3. So we dutifully run it on a single shard so it can produce the
"empty" aggs.
4. The shard we pick happens to not have the target of the range mapped.
5. This kicks in the special range aggregator that doesn't collect any
documents.
6. Before this commit, that range aggregator *also* never produced any
sub-aggs.
So, without this change, it was quite possible for a search that
happened to match no documents to "throw away" the sub-aggs of a range
and a few other aggs.
We've had this problem for a long, long time but it is more confusing
now because `can_match` is really kicking in and causing us to see cases
where it looks like you are targeting a lot of shards but you really are
only targeting a couple. It used to be that to get the "no sub-aggs"
behavior you had to explicitly target only shards that didn't map the
target field of the `range` agg. And, like, in that case it isn't too
bad because you targeted a sort of degenerate shard. But now that
`can_match` is doing its thing you can end up with the confusing steps
above. It took me several hours to track down what what happening I know
how the individual pieces of all of this works. It took four hours to
figure out how they fit together in this case....
Anyway! This replaces all the aggregator implementations that throw out
the sub-aggregators with ones that keep them. I think this'll be less
confusing in the future.
Closes#64142
0c47d49784