Lookup metrics percentiles using a float, not an integer.

The `metrics` filter was producing `0.0` for all percentiles except the 100th. I found [this post on the mailing list](https://groups.google.com/forum/#!topic/logstash-users/MxBDaWyzD9w) with the same issue.

This small change forces the parameter to `Snapshot.value()` to be a float rather than an integer. It seems to fix the issue for me in our setup. Unfortunately, the tests would not run on my Mac -- I assume this change to be simple enough to submit without running the tests.