Update metrics-3.0.xsd

Would like to create a new optional element into the tag in <metrics:annotation-driven> tag.

	<metrics:annotation-driven metric-registry="myMetrics" >
		<util:list id="customInterceptors" list-class="java.util.ArrayList" >
        	<value>com.myCustomMetrics.SystemExceptionMeteredInterceptor</value>
    	</util:list>
	</metrics:annotation-driven>