minor: make get_metrics.py executable

I forgot to add a shabang and mark get_metrics.py as an executable in
commit f17510c3672f ("get_metrics: rewrite in Python, refactor output a
bit").

Let's fix that.

Signed-off-by: Daniel Latypov <dlatypov@google.com>
Change-Id: I0bb6c2c6a0d671288bfdd02bf540ab57dbb028ff
diff --git a/get_metrics.py b/get_metrics.py
old mode 100644
new mode 100755
index 4f6fbb2..b744c4b
--- a/get_metrics.py
+++ b/get_metrics.py
@@ -1,3 +1,5 @@
+#!/usr/bin/env python3
+
 import argparse
 import subprocess
 from typing import Iterable, List, Tuple