Print Elastic APM trace ID from new transaction

printTraceID.go

// from new transaction
func Foo(){
  apmTx := apm.DefaultTracer.StartTransaction("transaction_name", "transaction_type")
	fmt.Println(apmTx.TraceContext().Trace.String()) // prints TraceID
	apmTx.End()
}

// from incoming http request
func FooHandler(w http.ResponseWriter, r *http.Request) {
	apmTx := apm.TransactionFromContext(r.Context())
	fmt.Println(apmTx.TraceContext().Trace.String()) // prints TraceID
	apmTx.End()
}