sstbatcher: pass unix nanos when computing stats

Previously, when regenerating stats from a split, we passed unix
seconds to the stats calculation routine instead of unix
nanoseconds. This is incorrect, but doesn't seem to have any observable
impact on behavior since the time is used to estimate the age of
garbage. The SST batcher cannot calculate garbage age bytes since it is
missing the previous value and therefore does not know the size of the
value.

Issue: none
Release note: none

Author: jeffswenson

pkg/kv/bulk/sst_adder.go

@@ -345,7 +345,7 @@ func addStatsToSplitTables(left, right, original *sstSpan, sendStartTimestamp ti
 		return err
 	}
 	statsIter.SeekGE(storage.MVCCKey{Key: right.start})
-	right.stats, err = storage.ComputeStatsForIter(statsIter, sendStartTimestamp.Unix())
+	right.stats, err = storage.ComputeStatsForIter(statsIter, sendStartTimestamp.UnixNano())
 	statsIter.Close()
 	if err != nil {
 		return err