tp: migrate sql metrics bazel script to use exec_tools

genrule.tools uses host mode and forces to be Python 2.

This CL corrects the use of the Python 3 tool so that it runs
Python 3 instead of Python 2 by using exec_tools.

Context (internal): see cl/326047210

Change-Id: I5bc1503eeb03a811b2ca0801a632c0249f71c851
diff --git a/BUILD b/BUILD
index 576dc0e..ec035d7 100644
--- a/BUILD
+++ b/BUILD
@@ -798,7 +798,7 @@
         "src/trace_processor/metrics/sql_metrics.h",
     ],
     cmd = "$(location gen_merged_sql_metrics_py) --cpp_out=$@ $(SRCS)",
-    tools = [
+    exec_tools = [
         ":gen_merged_sql_metrics_py",
     ],
 )