Added initial implementation of Topology-related event and
event metrics collector. It can be loaded by one of the following two (new)
features: onos-app-metrics, onos-app-metrics-topology

After loading the module, it subscribes for topology-related events
and keeps the following state:
 (a) The last 10 events
 (b) The timestamp of the last event (ms after epoch) as observed by this
     module
 (c) The rate of the topology events: count, median rate, average rate
     over the last 1, 5 or 15 minutes

The following CLI commands are added:
 * onos:topology-events
   Shows the last 10 topology events

 * onos:topology-events-metrics
   Shows the timestamp of the last event, and the rate of the topology
   events: see (b) and (c) above
diff --git a/apps/metrics/topology/pom.xml b/apps/metrics/topology/pom.xml
new file mode 100644
index 0000000..dc1c000
--- /dev/null
+++ b/apps/metrics/topology/pom.xml
@@ -0,0 +1,32 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
+  <modelVersion>4.0.0</modelVersion>
+
+  <parent>
+    <groupId>org.onlab.onos</groupId>
+    <artifactId>onos-app-metrics</artifactId>
+    <version>1.0.0-SNAPSHOT</version>
+    <relativePath>../pom.xml</relativePath>
+  </parent>
+
+  <artifactId>onos-app-metrics-topology</artifactId>
+  <packaging>bundle</packaging>
+
+  <description>ONOS topology metrics application</description>
+
+  <dependencies>
+    <dependency>
+      <groupId>org.onlab.onos</groupId>
+      <artifactId>onos-cli</artifactId>
+      <version>${project.version}</version>
+    </dependency>
+
+    <dependency>
+      <groupId>org.apache.karaf.shell</groupId>
+      <artifactId>org.apache.karaf.shell.console</artifactId>
+    </dependency>
+  </dependencies>
+
+</project>