Fix metrics for dispatched record size (#154)

In datastore, we use Length() to instrument payload size sent to them. However the value which was instrumented was the flatbuffer message. Fixing here to use the actual payload length

telemetry/record.go

@@ -110,12 +110,17 @@ func (record *Record) Raw() []byte {
 	return record.RawBytes
 }
 
-// Length gets the records byte size
-func (record *Record) Length() int {
+// Length gets the records flatbuffer payload byte size
+func (record *Record) LengthRawBytes() int {
 	record.ensureEncoded()
 	return len(record.RawBytes)
 }
 
+// Length gets the records byte size
+func (record *Record) Length() int {
+	return len(record.PayloadBytes)
+}
+
 // Encode encodes the records into bytes
 func (record *Record) Encode() ([]byte, error) {
 	record.ensureEncoded()