Add dashboard for redis keys (elastic#9746)

Add a dashboard with details for Redis keys using the new key metricset.
Visualizations included show lists length, what is useful when redis is used
for queuing, and average strings sizes and TTLs, what is useful when Redis
is used as a cache.

(cherry picked from commit ac04c32)

CHANGELOG.asciidoc

@@ -32,6 +32,8 @@ https://github.com/elastic/beats/compare/v6.6.0...6.x[Check the HEAD diff]
 - Fix issue preventing diskio metrics collection for idle disks. {issue}9124[9124] {pull}9125[9125]
 - Allow beats to blacklist certain part of the configuration while using Central Management. {pull}9099[9099]
 
+- Add `key` metricset to the Redis module. {issue}9582[9582] {pull}9657[9657] {pull}9746[9746]
+
 *Packetbeat*
 
 *Packetbeat*

metricbeat/docs/fields.asciidoc

@@ -17710,6 +17710,16 @@ type: keyword
 Key name.
 
 
+--
+
+*`redis.key.id`*::
++
+--
+type: keyword
+
+Unique id for this key (With the form <keyspace>:<name>).
+
+
 --
 
 *`redis.key.type`*::