Get Cloud Run env vars before calling metadata service for project ID

Checking the env vars is faster than calling the metadata service, so
hopefully this will bail out faster if we're not on Cloud Run.

Even if the call to the metadata service takes a long time it will only
happen on the first request to trigger it.

gaelog.go

@@ -80,18 +80,18 @@ func newServiceInfo() (serviceInfo, error) {
 		}, nil
 	}
 
-	// Try the metadata service for the project ID.
-	crProjectID, err := projectIDFromMetadataService()
-	if err != nil {
-		return serviceInfo{}, err
-	}
-
-	// We got the project ID, so get and check the env vars expected to be set on Cloud Run.
+	// Get and check the env vars expected to be set on Cloud Run.
 	crService := os.Getenv("K_SERVICE")
 	crRevision := os.Getenv("K_REVISION")
 	crConfiguration := os.Getenv("K_CONFIGURATION")
 	if crService == "" || crRevision == "" || crConfiguration == "" {
-		return serviceInfo{}, fmt.Errorf("gaelog: the project ID was fetched from the metadata service so $K_SERVICE, $K_REVISION, and $K_CONFIGURATION are expected to be set, but one or more are not. Falling back to standard library log.")
+		return serviceInfo{}, fmt.Errorf("gaelog: GAE env vars were not set so Cloud Run vars $K_SERVICE, $K_REVISION, and $K_CONFIGURATION are expected to be set, but one or more are not. Falling back to standard library log.")
+	}
+
+	// Finally, try the metadata service for the project ID.
+	crProjectID, err := projectIDFromMetadataService()
+	if err != nil {
+		return serviceInfo{}, err
 	}
 
 	return serviceInfo{

gaelog_test.go

@@ -69,8 +69,8 @@ func TestNew(t *testing.T) {
 		expectResource *monitoredres.MonitoredResource
 		expectErr      string
 	}{
-		{"no_env_vars_without_header", nil, false, nil, "the project ID was fetched from the metadata service"},
-		{"no_env_vars_with_header", nil, true, nil, "the project ID was fetched from the metadata service"},
+		{"no_env_vars_without_header", nil, false, nil, "GAE env vars were not set so Cloud Run vars"},
+		{"no_env_vars_with_header", nil, true, nil, "GAE env vars were not set so Cloud Run vars"},
 		{
 			"gae_env_vars_with_header",
 			map[string]string{