Collect metrics of Kafka consumer groups without active members

Nobody has claimed this yet.

`consumergroup` metricset of the Metricbeat `kafka` module collects consumer group metrics per member in the consumer group. When a consumer group doesn't have any active member, it doesn't collect any data, but this can be important to detect that all members are down, and to keep visualizing the last processed offsets.

Metricbeat should be able to collect metrics both, per consumergroup and per member.

I see two options for that:
* Split the consumergroup metricset in two, one for consumergroups, and another one for members. The one for members should take care of collecting the consumer lag for each member.
* In current consumergroup, generate an event for consumergroups that don't have any member, without the member-specific fields.

Example consumer group without members:
```
# ./kafka/bin/kafka-consumer-groups.sh --bootstrap-server localhost:9092 --command-config /kafka/bin/sasl-consumer.properties --describe --group foo-1
Picked up _JAVA_OPTIONS: -Djava.net.preferIPv4Stack=true
Consumer group 'foo-1' has no active members.

TOPIC PARTITION CURRENT-OFFSET LOG-END-OFFSET LAG CONSUMER-ID HOST CLIENT-ID
foo 0 1 2 1 - - -
foo 1 1 3 2 - - -
```

Start with the Metricbeat kafka module's consumergroup metricset and reproduce the no-active-members output shown in the issue. Compare the two proposed collection designs, then verify that consumer-group metrics are emitted without active members while member-specific lag remains available where applicable.

Written by the indexing model from the issue text.