Add step to enable the Redis PMDA (#4212)

(cherry picked from commit 522790d)

Author: OttaviaB

guides/common/modules/proc_configuring-pcp-data-collection.adoc

@@ -47,6 +47,12 @@ endif::[]
 # cd /var/lib/pcp/pmdas/postgresql
 # ./Install
 ----
+. Configure PCP to collect metrics from Redis:
++
+----
+# cd /var/lib/pcp/pmdas/redis
+# ./Install
+----
 . Enable the telemetry feature in {Project}:
 +
 [options="nowrap", subs="verbatim,quotes,attributes"]