Camera: Add exists function to CameraMetadata

Change-Id: I081f61417fca7c8408e9e767129647ddce5364ab
diff --git a/include/camera/CameraMetadata.h b/include/camera/CameraMetadata.h
index cf599e1..4289126 100644
--- a/include/camera/CameraMetadata.h
+++ b/include/camera/CameraMetadata.h
@@ -121,6 +121,12 @@
     }
 
     /**
+     * Check if a metadata entry exists for a given tag id
+     *
+     */
+    bool exists(uint32_t tag) const;
+
+    /**
      * Get metadata entry by tag id
      */
     camera_metadata_entry find(uint32_t tag);