add opentracing spans for metrics

vendor/github.com/Shopify/sarama/length_field.go

@@ -0,0 +1,29 @@
+package sarama
+
+import "encoding/binary"
+
+// LengthField implements the PushEncoder and PushDecoder interfaces for calculating 4-byte lengths.
+type lengthField struct {
+	startOffset int
+}
+
+func (l *lengthField) saveOffset(in int) {
+	l.startOffset = in
+}
+
+func (l *lengthField) reserveLength() int {
+	return 4
+}
+
+func (l *lengthField) run(curOffset int, buf []byte) error {
+	binary.BigEndian.PutUint32(buf[l.startOffset:], uint32(curOffset-l.startOffset-4))
+	return nil
+}
+
+func (l *lengthField) check(curOffset int, buf []byte) error {
+	if uint32(curOffset-l.startOffset-4) != binary.BigEndian.Uint32(buf[l.startOffset:]) {
+		return PacketDecodingError{"length field invalid"}
+	}
+
+	return nil
+}