feat: pre-existing app/transaction instance detection

end-to-end-tests/semi-instrumented/app-outside-main/expect.ref

@@ -0,0 +1,27 @@
+--- a/main.go
++++ b/main.go
+@@ -18,10 +18,13 @@
+ }
+
+ func noticeErrorWithAttributes(w http.ResponseWriter, r *http.Request) {
++	nrTxn := newrelic.FromContext(r.Context())
++
+ 	io.WriteString(w, "Noticing an error")
+
+ 	err := errors.New("error with attributes")
+ 	if err != nil {
++		nrTxn.NoticeError(err)
+ 		fmt.Println(err)
+ 	}
+ }
+@@ -41,8 +40,8 @@
+ 	}
+ 	myapp.WaitForConnection(5 * time.Second)
+
+-	http.HandleFunc("/", index)
+-	http.HandleFunc("/notice_error_with_attributes", noticeErrorWithAttributes)
++	http.HandleFunc(newrelic.WrapHandleFunc(myapp, "/", index))
++	http.HandleFunc(newrelic.WrapHandleFunc(myapp, "/notice_error_with_attributes", noticeErrorWithAttributes))
+
+ 	http.ListenAndServe(":8000", nil)
+ }

end-to-end-tests/semi-instrumented/app-outside-main/main.go

@@ -0,0 +1,48 @@
+// Copyright 2020 New Relic Corporation. All rights reserved.
+// SPDX-License-Identifier: Apache-2.0
+
+package main
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"net/http"
+	"time"
+
+	"github.com/newrelic/go-agent/v3/newrelic"
+)
+
+func index(w http.ResponseWriter, r *http.Request) {
+	io.WriteString(w, "hello world")
+}
+
+func noticeErrorWithAttributes(w http.ResponseWriter, r *http.Request) {
+	io.WriteString(w, "Noticing an error")
+
+	err := errors.New("error with attributes")
+	if err != nil {
+		fmt.Println(err)
+	}
+}
+
+func startNRApp() (*newrelic.Application, error) {
+	app, err := newrelic.NewApplication(
+		newrelic.ConfigAppName("Example App"),
+		newrelic.ConfigFromEnvironment(),
+	)
+	return app, err
+}
+
+func main() {
+	myapp, err := startNRApp()
+	if err != nil {
+		panic(err)
+	}
+	myapp.WaitForConnection(5 * time.Second)
+
+	http.HandleFunc("/", index)
+	http.HandleFunc("/notice_error_with_attributes", noticeErrorWithAttributes)
+
+	http.ListenAndServe(":8000", nil)
+}