tp: switch merged metrics script to python3

Change-Id: I2765179dc7851b6a5c59c4a166c1e06be8beda32

tools/gen_merged_sql_metrics.py

diff --git a/tools/gen_merged_sql_metrics.py b/tools/gen_merged_sql_metrics.py
index 07c57e1..ca2c525 100755
--- a/tools/gen_merged_sql_metrics.py
+++ b/tools/gen_merged_sql_metrics.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python
+#!/usr/bin/env python3
 # Copyright (C) 2019 The Android Open Source Project
 #
 # Licensed under the Apache License, Version 2.0 (the "License");