Add parameter to optionally strip deploy path.

[lapo-luchini]

Closes #631.

[fstab]

Thanks a lot or the PR. Here are a few minor comments:

Please update the class JavaDoc for MetricsFilter. Add the new option with its default to the example:

    <!-- strip-context-path is optional, defaults to false -->
   <init-param>
      <param-name>strip-context-path</param-name>
      <param-value>false</param-value>
   </init-param>

...and add an explanation of the strip-context-path option to the text above the example.

Additionally, please also update the Servlet Filter section in README.md.

There should be a test for the new option in MetricsFilterTest. With copy-and-paste from init() it could look something like this

    @Test
    public void testStripContextPath() throws Exception {
        String metricName = "foo";
        FilterConfig cfg = mock(FilterConfig.class);
        when(cfg.getInitParameter(MetricsFilter.METRIC_NAME_PARAM)).thenReturn(metricName);
        when(cfg.getInitParameter(MetricsFilter.PATH_COMPONENT_PARAM)).thenReturn("0");
        when(cfg.getInitParameter(MetricsFilter.STRIP_CONTEXT_PATH_PARAM)).thenReturn("true");
        f.init(cfg);
        assertTrue(f.stripContextPath);
        HttpServletRequest req = mock(HttpServletRequest.class);
        when(req.getRequestURI()).thenReturn("/foo/bar/baz/bang");
        when(req.getContextPath()).thenReturn("/foo/bar");
        when(req.getMethod()).thenReturn(HttpMethods.GET);
        HttpServletResponse res = mock(HttpServletResponse.class);
        FilterChain c = mock(FilterChain.class);
        f.doFilter(req, res, c);
        verify(c).doFilter(req, res);
        final Double sampleValue = CollectorRegistry.defaultRegistry.getSampleValue(metricName + "_count", new String[]{"path", "method"}, new String[]{"/baz/bang", HttpMethods.GET});
        assertNotNull(sampleValue);
        assertEquals(1, sampleValue, 0.0001);
    }

[lapo-luchini]

@fstab let me know if anything more should be changed.

[fstab]

Thanks a lot @lapo-luchini, and sorry that it took so long to merge this.