Added request counter to Paginator

Added instrumentation for Paginator so API clients can understand the number of requests executed per search.
I guess the reason I suggested that is because I'm not sure if it should be the responsibility of Paginator to track this information.

I can suggest another way to count successful request that doesn't modify Paginator: Pass the .create() method an instance of a Genomics stub that was constructed from a custom HttpRequestInitializer that counts the number of requests made, and use a custom HttpUnsuccessfulResponseHandler to count the number of requests that fail. The difference between the numbers is the number of successful requests.