metrics: fix registration on Windows

We always want to use / as the seperator to access VFS paths in metrics
not \ even on Windows.

Change-Id: Ia2670d0027c79dda3c5ccfe235dfb230e0414281
diff --git a/tools/gen_merged_sql_metrics.py b/tools/gen_merged_sql_metrics.py
index ca2c525..065ff1b 100755
--- a/tools/gen_merged_sql_metrics.py
+++ b/tools/gen_merged_sql_metrics.py
@@ -108,7 +108,7 @@
       variable = filename_to_variable(os.path.splitext(name)[0])
 
       # This is for Windows which has \ as a path separator.
-      path = path.replace("\\", "\\\\")
+      path = path.replace("\\", "/")
       output.write('\n  {{"{}", {}}},\n'.format(path, variable))
     output.write("};\n")