Include JCIP annotations for fundamentals types.

For some of the base metric types, we already had Java Concurrency in
Practice annotations, but they were not comprehensively canvassed.  In
the interests of promoting Javadoc documentation that is completely
self-contained, they should continue to be used.
Merge pull request #19 from prometheus/feature/documentation/jcip-annotations

Include JCIP annotations for fundamentals types.