records.lag should use tags for topic and partition rather than using metric name.

As part of KIP-92[1] a per partition lag metric was added.

These metrics are really useful, however in the implementation it was implemented as a prefix to the metric name: https://github.com/apache/kafka/blob/trunk/clients/src/main/java/org/apache/kafka/clients/consumer/internals/Fetcher.java#L1321-L1344

Usually these kind of metrics use tags and the name is constant for all topics, partitions.

We have a custom reporter which aggregates topics/partitions together to avoid explosion of the number of KPIs and this KPI doesn't support this as it doesn't have tags but a complex name.

[1] https://cwiki.apache.org/confluence/display/KAFKA/KIP-92+-+Add+per+partition+lag+metrics+to+KafkaConsumer