Metricbeat Redis module: add length of lists in Redis metrics

[tbragin]

This is a request for adding to the Redis Metricbeat module the length of lists in Redis, a metric that represents the length of each list (as in the collectd example), or all lists in the key as a total (as in the prometheus/grafana example).

With collectd it is possible to issue queries about the length of lists.

  <Plugin redis>
    <Node "example">
        Host "localhost"
        Port "6379"
        Timeout 2000
        <Query "LLEN myqueue">
          Type "queue_length"
          Instance "myqueue"
        <Query>
    </Node>
  </Plugin>

Prometheus/Graphana also provides the total items per key: https://github.com/oliver006/redis_exporter

We are only gathering info about the number of keys, but no information about the length of lists in keys.
https://www.elastic.co/guide/en/beats/metricbeat/current/metricbeat-metricset-redis-keyspace.html

    "redis": {
        "keyspace": {
            "avg_ttl": 0,
            "expires": 0,
            "id": "db0",
            "keys": 1
        }
    }

cc: @jsoriano

[elasticmachine]

Pinging @elastic/infrastructure