Fix percentiles on metrics filter

I introduced a bug on https://github.com/logstash/logstash/commit/acc8247accdf09c912ddb90afa1c7131e374e3b5, row 199.

The integer division of any percentile, like 90 by 100, is always 0.

This pull request fixes the problem and includes a regression test.

I'm really sorry for that.