Add version field to LevelDB sample format.

This doesn't add complex discriminator logic yet, but adds a single
version byte to the beginning of each samples chunk. If we ever need to
change the disk format again, this will make it easy to do so without
having to wipe the entire database.

Change-Id: I60c39274256f790bc2da83167a1effaa174588fe

storage/metric/sample.go

@@ -23,8 +23,14 @@ import (
 	clientmodel "github.com/prometheus/client_golang/model"
 )
 
-// bytesPerSample is the number of bytes per sample in marshalled format.
-const bytesPerSample = 16
+const (
+	// sampleSize is the number of bytes per sample in marshalled format.
+	sampleSize = 16
+	// formatVersion is used as a version marker in the marshalled format.
+	formatVersion = 1
+	// formatVersionSize is the number of bytes used by the serialized formatVersion.
+	formatVersionSize = 1
+)
 
 // MarshalJSON implements json.Marshaler.
 func (s SamplePair) MarshalJSON() ([]byte, error) {
@@ -153,9 +159,10 @@ func (v Values) String() string {
 
 // marshal marshals a group of samples for being written to disk.
 func (v Values) marshal() []byte {
-	buf := make([]byte, len(v)*bytesPerSample)
+	buf := make([]byte, formatVersionSize+len(v)*sampleSize)
+	buf[0] = formatVersion
 	for i, val := range v {
-		offset := i * 16
+		offset := formatVersionSize + i*sampleSize
 		binary.LittleEndian.PutUint64(buf[offset:], uint64(val.Timestamp.Unix()))
 		binary.LittleEndian.PutUint64(buf[offset+8:], math.Float64bits(float64(val.Value)))
 	}
@@ -164,14 +171,17 @@ func (v Values) marshal() []byte {
 
 // unmarshalValues decodes marshalled samples and returns them as Values.
 func unmarshalValues(buf []byte) Values {
-	n := len(buf) / bytesPerSample
+	n := len(buf) / sampleSize
 	// Setting the value of a given slice index is around 15% faster than doing
 	// an append, even if the slice already has the required capacity. For this
 	// reason, we already set the full target length here.
 	v := make(Values, n)
 
+	if buf[0] != formatVersion {
+		panic("unsupported format version")
+	}
 	for i := 0; i < n; i++ {
-		offset := i * 16
+		offset := formatVersionSize + i*sampleSize
 		v[i] = &SamplePair{
 			Timestamp: clientmodel.TimestampFromUnix(int64(binary.LittleEndian.Uint64(buf[offset:]))),
 			Value:     clientmodel.SampleValue(math.Float64frombits(binary.LittleEndian.Uint64(buf[offset+8:]))),