Expose per-region statistics for Hibernate L2 cache

[kennymacleod]

As of Micrometer 1.5, the HibernateMetrics binder exposes L2 cache statistics from the org.hibernate.stat.Statistics interface. This is useful, but it would be better if this were done on a per-cache-region basis, giving us more fine-grained per-entity and per-collection information.

This can be done by using getSecondLevelCacheRegionNames and getSecondLevelCacheStatistics, giving you a SecondLevelCacheStatistics object.

As well as providing the same hit/miss/put stats you get from the higher-level Statistics interface, you also get cache size and memory usage stats. These can then be tagged with the region name.

[jkschneider]

Nice! Let's do this.

[shakuzen]

I'm not familiar with L2 cache regions. Are these static or dynamic? Will we know at binding time what all the regions are, or could they change?

[kennymacleod]

Good question, @shakuzen. My assumption was that once the SessionFactory is up and running, the set of regions is immutable. I just had a rummage through the Hibernate 5.2 codebase, and this does indeed seem to be the case. While the set of regions is mutable (for addition only) in the Statistics implementation class, this only occurs in the SessionFactoryImpl constructor. After that, there's no way to change them.