out_stackdriver: add missing / to URI

Without this, the plugin generates an invalid request with a relative
path.

Signed-off-by: Rodrigo Queiro <[email protected]>

Author: drigz

plugins/out_stackdriver/gce_metadata.h

@@ -23,7 +23,7 @@
 #include "stackdriver.h"
 
 /* Project ID metadata URI */
-#define FLB_STD_METADATA_PROJECT_ID_URI "computeMetadata/v1/project/numeric-project-id"
+#define FLB_STD_METADATA_PROJECT_ID_URI "/computeMetadata/v1/project/numeric-project-id"
 
 /* Zone metadata URI */
 #define FLB_STD_METADATA_ZONE_URI "/computeMetadata/v1/instance/zone"