Most aggregations (terms, histogram, stats, percentiles, geohash-grid) now
support a new `missing` option which defines the value to consider when a
field does not have a value. This can be handy if you eg. want a terms
aggregation to handle the same way documents that have "N/A" or no value
for a `tag` field.
This works in a very similar way to the `missing` option on the `sort`
element.
One known issue is that this option sometimes cannot make the right decision
in the unmapped case: it needs to replace all values with the `missing` value
but might not know what kind of values source should be produced (numerics,
strings, geo points?). For this reason, we might want to add an `unmapped_type`
option in the future like we did for sorting.
Related to #5324
32e23b9100