Add clear_interval and flush_interval options for metrics filter
Add observation that intervals should be a multiple of 5s
Adds tests for metrics filter and fixes key collision

When using multiple instances of the metrics filter,
the keys collided, making the counters interfere with
each other.
Add metrics filter tests for clear and flush intervals
Merge pull request #545 from jbochi/master

Add clear_interval and flush_interval options for metrics filter