[dbnode] wrong metrics calculation for block iterator

[asafm]

@ryanhall07 @robskillington Just verifying with you as I read this code multiple times, and I think I found a bug:
size is the number of elements in results as effectively returned originally here:

func (r *results) AddDocuments(batch []doc.Document) (int, int, error) {
	r.Lock()
	err := r.addDocumentsBatchWithLock(batch)
	size := r.resultsMap.Len()
	docsCount := r.totalDocsCount + len(batch)
	r.totalDocsCount = docsCount
	r.Unlock()
	return size, docsCount, err
}

Since results only keeps growing, as I haven't seen any reset to this object, it means that when you call iter.Counts() to get the seriesMatched, you will get a wrong number, since the AddSeries(size) keeping adding up total and not adding delta. Maybe the correct way is to measure results before adding and after to get that delta?

Originally posted by @asafm in #3269 (comment)