Add option to always collect metrics for all topics

[graphex]

As mentioned in #66, instead of only collecting topic metrics for topic partitions with an active consumer, sometimes it is important to collect metrics for topics without an active consumer, so that unconsumed partition alerts can be generated.

[seglo]

Good idea.

[seglo]

Please leave a comment if anyone is interested in picking this up.

[KarstenWintermann]

@seglo I would like to generate alerts for new messages in dead letter queues.

I initially thought I could do that based on the difference of kafka_partition_earliest_offset and kafka_partition_latest_offset, but then realized that they aren't reported because my dead letter queues don't have consumers.

Is there an easier approach for generating alerts like that? Otherwise I would be in favor of having the functionality described above.