Decouple MetricsFilter from Spring MVC

[wilkinsona]

MetricsFilter uses Spring MVC's BEST_MATCHING_PATTERN_ATTRIBUTE request attribute to determine the metric name for a request. This couples MetricsFilter to Spring MVC unnecessarily and causes problems with other web frameworks such as Jersey (see #5875).

I think it would be better if MetricsFilter defined its own request attribute and specified the format of its value. This would allow web frameworks to map from whatever format they use for url templates to something that's suitable for us in a metric name. We could provide an implementation for Spring MVC and, to fix #5875, an implementation for Jersey too.