Add a metric for plain topic offsets

[hchargois]

This adds a new metric kafka_burrow_topic_partition_offset that gives the offsets of topics (by partition). This allows monitoring topic growth, even if the topic is not currently consumed by a consumer group (all other metrics have to do with consumer groups).

[beorn-]

:+1:

[jirwin]

This looks good to me. I'll give it a whirl soon so we can get this merged.

[hchargois]

Hi @jirwin ! Have you had time to test this? :)