Add support for async requests in MetricsFilter

It seems to me that [MetricsFilter](https://github.com/prometheus/client_java/blob/master/simpleclient_servlet/src/main/java/io/prometheus/client/filter/MetricsFilter.java) currently does not support async requests, it reports very low times which I guess are only limited to the time the initial request is answered.