Retrieves project id from environment var (#632)

example/metric/README.md

@@ -16,6 +16,12 @@ foo-project-214354
 
 In this case, the project ID is `foo-project-214354`.
 
+Next, set this project ID to the environment variable `GOOGLE_PROJECT_ID` using the following command:
+
+```
+export GOOGLE_PROJECT_ID=$(gcloud config get-value project)
+```
+
 Once you ensure the API is enabled, then build the example application and run the executable.
 
 ```

example/metric/example.go

@@ -18,6 +18,7 @@ import (
 	"context"
 	"log"
 	"math/rand"
+	"os"
 	"sync"
 	"time"
 
@@ -55,7 +56,7 @@ func main() {
 	// Initialization. In order to pass the credentials to the exporter,
 	// prepare credential file following the instruction described in this doc.
 	// https://pkg.go.dev/golang.org/x/oauth2/google?tab=doc#FindDefaultCredentials
-	exporter, err := mexporter.New()
+	exporter, err := mexporter.New(mexporter.WithProjectID(os.Getenv("GOOGLE_PROJECT_ID")))
 	if err != nil {
 		log.Fatalf("Failed to create exporter: %v", err)
 	}