Metrics from Caffeine caches

[john-karp]

Add ability to gather metrics from Caffeine cache library. An instance of MetricsStatsCounter should be passed to the Caffeine cache builder, resulting in Caffeine using that class to publish metrics to the desired metrics registry. (An example of this usage is in the UT.)

Code was taken from example given by Caffeine library author provided at https://github.com/ben-manes/caffeine/blob/master/examples/stats-metrics/src/test/java/com/github/benmanes/caffeine/examples/stats/metrics/MetricsStatsCounterTest.java

Adding it into official dropwizard metrics repo should make it easier for dependent projects to take advantage of.

It has the Apache Public License 2.0, same as this project, so I assume it is okay to integrate?

Changes made to copied sources:

• Package names made consistent rest of project
• Switched UT to use JUnit, added more UT
• Used Histogram/Timer where possible
• Break down evictions by RemovalCause
• Documentation

[john-karp]

@ben-manes see any reason not to?

[ben-manes]

The only limitation is sometimes users add cache.estimatedSize, e.g. CaffeineCacheMetrics in Micrometer. I am not sure why that's desired unless one is using an unbounded cache, at which point there's inherent risk in production. The Dropwizard Ehcache2 metrics have an objects for the cache size, whereas the JCache one does not. Ideally size would be flat when it reaches the logical or bounded size threshold and not offer a meaningful devops data point.

Based on that, I think the push model is nice though polling like Guava's CacheStats is also an option. You are welcome to take that example code, or not, as you see fit.

[pkwarren]

Changes look good to me.

[joschi]

LGTM in general.

Would you please add a page in the documentation similar to the EhCache documentation (https://metrics.dropwizard.io/4.1.2/manual/ehcache.html)?

[arteam]

LGTM! Looking forward for Caffeine metrics.

[john-karp]

@joschi - added documentation

[joschi]

@john-karp @ben-manes LGTM, thanks a lot!

[ben-manes]

Thank you @john-karp for taking my simple example and making it production worthy! :)