Only include annotations instead of full Stagemonitor dependency

Stagemonitor and its dependencies are no longer included in builds. To
enable Stagemonitor in production, the libraries must be provided by the
host system. Additionally, the Java Servlet container needs to be setup
to load these additional dependencies for the web app.

For Tomcat 8 this is done by setting up a context config file in
conf/Catalina/localhost/<context path>.xml with the following content:

    <Context>
      <Resources className="org.apache.catalina.webresources.StandardRoot">
        <JarResources className="org.apache.catalina.webresources.DirResourceSet"
            base="<path to Stagemonitor JARs>"
            webAppMount="/WEB-INF/lib">
        </JarResources>
      </Resources>
    </Context>

See https://tomcat.apache.org/tomcat-8.0-doc/config/resources.html for
more configuration options.

pom.xml

@@ -293,9 +293,9 @@
 			<version>1.5.12</version>
 		</dependency>
 		<dependency>
-			<groupId>org.stagemonitor</groupId>
-			<artifactId>stagemonitor-web</artifactId>
-			<version>0.31.0</version>
+			<groupId>com.codahale.metrics</groupId>
+			<artifactId>metrics-annotation</artifactId>
+			<version>3.0.2</version>
 		</dependency>
 	</dependencies>
 

src/main/java/org/stagemonitor/core/metrics/MonitorGauges.java

+package org.stagemonitor.core.metrics;
+
+import java.lang.annotation.ElementType;
+import java.lang.annotation.Retention;
+import java.lang.annotation.RetentionPolicy;
+import java.lang.annotation.Target;
+
+/**
+ * When a type is marked with this annotation, the creation of gauges with
+ * @{@link com.codahale.metrics.annotation.Gauge} is activated for that type.
+ *
+ * <pre><code>
+ *     \@MonitorGauges
+ *     public class Queue {
+ *         \@Gauge(name = "queueSize")
+ *         public int getQueueSize() {
+ *             return queue.size;
+ *         }
+ *     }
+ * </code></pre>
+ */
+@Target(ElementType.TYPE)
+@Retention(RetentionPolicy.RUNTIME)
+public @interface MonitorGauges {
+}