Add destroy() method to MetricsServlet

[ogafosir]

The MetricsServlet keeps connections open, as it never closes the response. This cause problems in some containers as they are having problems taking the Servlet out of service. This may eventually take so much time, that operating the restart button form within the console no longer function properly.

This may be solved by adding a destroy method as discussed here:

See: https://groups.google.com/forum/?fromgroups=#!topic/hystrixoss/5exghYCppG0

Will submit a pull request eventually...

[benjchristensen]

This is released in 1.2.13