Added request counter to Paginator

Added instrumentation for Paginator so API clients can understand the number of requests executed per search.
Thanks for the changes Kelly!  I take it `v1beta2-0.10-SNAPSHOT` is basically the same as `v1beta2-0.9` based on [line 11 in pom.xml](https://github.com/googlegenomics/utils-java/blob/master/pom.xml#L11)?