TSDB: eliminate one yolostring

When the only use of a []byte->string conversion is as a map key, Go
doesn't allocate.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

tsdb/index/index.go

@@ -612,7 +612,7 @@ func (w *Writer) writeLabelIndices() error {
 		cnt--
 		d.Uvarint()               // Keycount.
 		name := d.UvarintBytes()  // Label name.
-		value := yoloString(d.UvarintBytes()) // Label value.
+		value := d.UvarintBytes() // Label value.
 		d.Uvarint64()             // Offset.
 		if len(name) == 0 {
 			continue // All index is ignored.
@@ -626,7 +626,7 @@ func (w *Writer) writeLabelIndices() error {
 			values = values[:0]
 		}
 		current = name
-		sid, ok := w.symbolCache[value]
+		sid, ok := w.symbolCache[string(value)]
 		if !ok {
 			return fmt.Errorf("symbol entry for %q does not exist", string(value))
 		}