Specify the base name of the metrics as a filter init-param for the metrics captured in the AbstractInstrumentedFilter

This PR adds the ability to specify the base name of the metrics collected by an instance of the AbstractInstrumentedFilter as a filter init-param, `prefix-name`. If no init-param is specified, it will fall back to the name of the filter class.

This is also handy in the case where multiple instances of the InstrumentedFilter need to be used to instrument different url-patterns. An example is seen below:

```
<filter>
        <filter-name>instrumentedFilter</filter-name>
        <filter-class>com.codahale.metrics.servlet.InstrumentedFilter</filter-class>
        <init-param>
            <param-name>name-prefix</param-name>
            <param-value>foo</param-value>
        </init-param>
    </filter>
    <filter-mapping>
        <filter-name>instrumentedFilter</filter-name>
        <url-pattern>/foo/*</url-pattern>
    </filter-mapping>
   <filter>
        <filter-name>instrumentedFilter2</filter-name>
        <filter-class>com.codahale.metrics.servlet.InstrumentedFilter</filter-class>
        <init-param>
            <param-name>name-prefix</param-name>
            <param-value>bar</param-value>
        </init-param>
    </filter>
    <filter-mapping>
        <filter-name>instrumentedFilter2</filter-name>
        <url-pattern>/bar/*</url-pattern>
    </filter-mapping>
```