Tracking the key metrics over time is very helpful in understanding the cluster and user behavior.
Currently even basic metrics tracking requires setting up an external system and either polling /admin/metrics or using SolrMetricReporter-s. The advantage of this setup is that these external tools usually provide a lot of sophisticated functionality. The downside is that they don't ship out of the box with Solr and require additional admin effort to set up.
Solr could collect some of the key metrics and keep their historical values in a round-robin database (eg. using RRD4j) to keep the size of the historic data constant (eg. ~64kB per metric), but at the same providing out of the box useful insights into the basic system behavior over time. This data could be persisted to the .system collection as blobs, and it could be also presented in the Admin UI as graphs.