MB-60914: Zap field specific metrics (#38)

 - Added a new interface called FieldStatsReporter which has a
function called UpdateFieldStats
 - This function is expected to be implemented by SegmentBase and
can be called after it is created to populate a map with field stats
 - Currently this is called after merge to find out the number of
vectors in the newly created segment

segment.go

@@ -168,3 +168,13 @@ type DocVisitState interface {
 type StatsReporter interface {
  ReportBytesWritten(bytesWritten uint64)
 }
+
+type FieldStatsReporter interface {
+ UpdateFieldStats(FieldStats)
+}
+
+type FieldStats interface {
+ Store(statName, fieldName string, value uint64)
+ Aggregate(stats FieldStats)
+ Fetch() map[string]map[string]uint64
+}