metrics: Use jq tool to pretty-print json metrics output

This PR enables the use of jq pretty-print feature to
improve the formatting of metric results json files.

Fixes: #8081

Signed-off-by: David Esparza <david.esparza.borquez@intel.com>
(cherry picked from commit 8c498ef)

tests/metrics/lib/json.bash

@@ -147,7 +147,7 @@ done)
 EOF
 )"
 
-	echo "$json" > $json_filename
+	echo "${json}" | jq . > "${json_filename}"
 
 	# If we have a JSON URL or host/socket pair set up, post the results there as well.
 	# Optionally compress into a single line.