MB-60914: Zap field specific metrics

[Likith101]

• Added an UpdateFieldStats function which will populate the passed map with any field related metrics
• The map is expected to be already initialized with the appropriate metric names
• As of now, the only metric added is num_vectors which is obtained by looking at its appropriate field section data
• This call also is only necessary after a merge because otherwise we can obtain it from the analysis results.
• With the inclusion of UpdateFieldStats, SegmentBase will now implement the FieldStatsReporter defined in scorch_segment_api