Add AddTraceHeaders helper function

README.md

@@ -2,7 +2,7 @@
 
 ## Usage
 
-Datadog needs to be able to read context from the incoming Lambda event, in order to enable distributed tracing.
+Datadog needs to be able to read headers from the incoming Lambda event, in order to add datadog metadata to the go context.
 Wrap your lambda handler like so.
 
 ```go
@@ -13,11 +13,19 @@ import (
 	"github.com/DataDog/dd-lambda-go"
 )
 
-func myHandler() (string, error) {
-	return "Success", nil
-}
-
 func main() {
+	// Wrap your lambda handler like this
     lambda.Start( ddlambda.WrapHandler(myHandler))
 }
+
+func myHandler(ctx context.Context, event MyEvent) (string, error) {
+	// Add headers to outbound using the context object
+	req, err := http.NewRequest("GET", "http://api.youcompany.com/status")
+	ddlambda.AddTraceHeaders(ctx)
+
+	client := http.Client{}
+	client.Do(req)
+
+	return "Success", nil
+}
 ```

ddlambda.go

@@ -3,6 +3,7 @@ package ddlambda
 import (
 	"context"
 	"encoding/json"
+	"net/http"
 
 	"github.com/DataDog/dd-lambda-go/internal/trace"
 )
@@ -24,6 +25,14 @@ func GetTraceHeaders(ctx context.Context) map[string]string {
 	return result
 }
 
+// AddTraceHeaders adds DataDog trace headers to a HTTP Request
+func AddTraceHeaders(req *http.Request) {
+	headers := GetTraceHeaders(req.Context())
+	for key, value := range headers {
+		req.Header.Add(key, value)
+	}
+}
+
 func (hl *handlerListener) HandlerStarted(ctx context.Context, msg json.RawMessage) context.Context {
 	ctx, _ = trace.ExtractTraceContext(ctx, msg)
 	return ctx