This change returns the total number of fields at the segment level,
allowing for a more accurate estimate of the memory used by Lucene. The
new estimate is expected to be closer to the actual memory usage than
the current estimate using the index-level field count, due to the
non-trivial overhead incurred by each Lucene segment. Two new fields are
introduced: total_segment_fields, which is the total number of fields at
the segment level, and average_fields_per_segment. The overhead per
field in segments with fewer fields is larger than in segments with many
fields.
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