stringlabels: size buffer for added labels

This makes the buffer the correct size for the common case that labels
have only been added. It will be too large for the case that labels are
changed, but the current buffer resize logic in `appendLabelTo` doubles
the buffer, so a small over-estimate is better.

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

model/labels/labels_string.go

@@ -599,7 +599,8 @@ func (b *Builder) Labels(res Labels) Labels {
 	sort.Strings(b.del)
 	a, d := 0, 0
 
-	buf := make([]byte, 0, len(b.base.data)) // TODO: see if we can re-use the buffer from res.
+	bufSize := len(b.base.data) + labelsSize(b.add)
+	buf := make([]byte, 0, bufSize) // TODO: see if we can re-use the buffer from res.
 	for pos := 0; pos < len(b.base.data); {
 		oldPos := pos
 		var lName string