Use awk instead of sed when parsing du output (#112)

This feel a little less "magic", is better readable and maintainable. Might have a slight impact on performance, too.

directory-size.sh

@@ -13,4 +13,4 @@
 echo "# HELP node_directory_size_bytes Disk space used by some directories"
 echo "# TYPE node_directory_size_bytes gauge"
 du --block-size=1 --summarize "$@" \
-  | sed -ne 's/\\/\\\\/;s/"/\\"/g;s/^\([0-9]\+\)\t\(.*\)$/node_directory_size_bytes{directory="\2"} \1/p'
+  | awk '{ print "node_directory_size_bytes{directory=\"" $2 "\"} "  $1 }'