Uri recently pushed a commit to use it in order to do the initial sizing of data-structures for aggregations: https://github.com/elasticsearch/elasticsearch/commit/7b13f1932e28b24ba54703216e59433715814b1e But we cap it anyway: the query may only see a few values, and even when there are lots of unique values it may be faster to grow on demand.

Would `-1` really be a better value when unknown? I think the common use for this number is to return the maximum cardinality of a segment. If `-1` is the value meaning unknown, we would need to compute something like:
```
long maxCardinality = 0;
for (AtomicReaderContext readerContext : topLevelReader.leaves()) {
    AtomicFieldData fd = [...];
    if (fd.getNumberUniqueValues() == -1) {
      return -1;
    } else {
      maxCardinality = Math.max(fd.getNumberUniqueValues(), maxCardinality);
    }
}
return maxCardinality;
```
On the other hand, with MAX_VALUE meaning unknown, it becomes simpler:
```
long maxCardinality = 0;
for (AtomicReaderContext readerContext : topLevelReader.leaves()) {
    AtomicFieldData fd = [...];
    maxCardinality = Math.max(fd.getNumberUniqueValues(), maxCardinality);
}
return maxCardinality;
```