Expose how many bytes are written in ProtoOutputStream

Bug: 69428467
Test: N/A
Change-Id: If1405885949635cebd1c6d89e220c210ef9fa210
diff --git a/libs/protoutil/include/android/util/ProtoOutputStream.h b/libs/protoutil/include/android/util/ProtoOutputStream.h
index ce41849..faea9b2 100644
--- a/libs/protoutil/include/android/util/ProtoOutputStream.h
+++ b/libs/protoutil/include/android/util/ProtoOutputStream.h
@@ -110,6 +110,12 @@
     void end(long long token);
 
     /**
+     * Returns how many bytes are buffered in ProtoOutputStream.
+     * Notice, this is not the actual(compact) size of the output data.
+     */
+    size_t bytesWritten();
+
+    /**
      * Flushes the protobuf data out to given fd. When the following functions are called,
      * it is not able to write to ProtoOutputStream any more since the data is compact.
      */