Including the IO stats as part of metadata (#1774)

* setting the total bytes written as an internal key in bolt

* bug fix, nil pointer deref

index/scorch/persister.go

@@ -543,6 +543,13 @@ func prepareBoltSnapshot(snapshot *IndexSnapshot, tx *bolt.Tx, path string,
 		}
 	}
 
+	if snapshot.parent != nil {
+		val := make([]byte, 8)
+		bytesWritten := atomic.LoadUint64(&snapshot.parent.stats.TotBytesWrittenAtIndexTime)
+		binary.LittleEndian.PutUint64(val, bytesWritten)
+		internalBucket.Put(TotBytesWrittenKey, val)
+	}
+
 	var filenames []string
 	newSegmentPaths := make(map[uint64]string)
 
@@ -697,6 +704,7 @@ var boltMetaDataKey = []byte{'m'}
 var boltMetaDataSegmentTypeKey = []byte("type")
 var boltMetaDataSegmentVersionKey = []byte("version")
 var boltMetaDataTimeStamp = []byte("timeStamp")
+var TotBytesWrittenKey = []byte("TotBytesWritten")
 
 func (s *Scorch) loadFromBolt() error {
 	return s.rootBolt.View(func(tx *bolt.Tx) error {