* Added per-field size details.
This also does a bunch of refactoring.
merging field metadata does not silently asserts that arguments should be sorted.
merging does not set `stored`.
We do not rely on a hashmap to group fields, but instead rely on the fact that
the term dictionary is sorted.
The inverted level method that exposes field metadata is not exposed
as public anymore.
* CR comment
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Co-authored-by: Paul Masurel <paul.masurel@datadoghq.com>