Added a new class EventMetric that can be used for measuring events:

rate and last event timestamp (ms from epoch)

Change-Id: Ifce6c2ca911f3b288a0efd9061befc936d06da51

+package org.onlab.metrics;
+
+import com.codahale.metrics.Gauge;
+import com.codahale.metrics.Meter;
+
+/**
+ * Metric measurements for events.
+ */
+public class EventMetric {
+    private static final String GAUGE_TIMESTAMP_NAME = "Timestamp.EpochMs";
+    private static final String METER_RATE_NAME = "Rate";
+
+    private final MetricsService metricsService;
+    private final String componentName;
+    private final String featureName;
+
+    private MetricsComponent metricsComponent;
+    private MetricsFeature metricsFeature;
+
+    private volatile long lastEventTimestampEpochMs = 0;
+    private Gauge<Long> lastEventTimestampGauge;
+    private Meter eventRateMeter;
+
+    /**
+     * Constructor.
+     *
+     * @param metricsService the Metrics Service to use for Metrics
+     * registration and deregistration
+     * @param componentName the Metrics Component Name to use for Metrics
+     * registration and deregistration
+     * @param featureName the Metrics Feature Name to use for Metrics
+     * registration and deregistration
+     */
+    public EventMetric(MetricsService metricsService, String componentName,
+                       String featureName) {
+        this.metricsService = metricsService;
+        this.componentName = componentName;
+        this.featureName = featureName;
+    }
+
+    /**
+     * Registers the metrics.
+     */
+    public void registerMetrics() {
+        metricsComponent = metricsService.registerComponent(componentName);
+        metricsFeature = metricsComponent.registerFeature(featureName);
+
+        lastEventTimestampEpochMs = 0;
+        lastEventTimestampGauge =
+            metricsService.registerMetric(metricsComponent,
+                                          metricsFeature,
+                                          GAUGE_TIMESTAMP_NAME,
+                                          new Gauge<Long>() {
+                                              @Override
+                                              public Long getValue() {
+                                                  return lastEventTimestampEpochMs;
+                                              }
+                                          });
+
+        eventRateMeter = metricsService.createMeter(metricsComponent,
+                                                    metricsFeature,
+                                                    METER_RATE_NAME);
+    }
+
+    /**
+     * Removes the metrics.
+     */
+    public void removeMetrics() {
+        lastEventTimestampEpochMs = 0;
+        metricsService.removeMetric(metricsComponent,
+                                    metricsFeature,
+                                    GAUGE_TIMESTAMP_NAME);
+        metricsService.removeMetric(metricsComponent,
+                                    metricsFeature,
+                                    METER_RATE_NAME);
+    }
+
+    /**
+     * Updates the metric measurements for a single event.
+     */
+    public void eventReceived() {
+        lastEventTimestampEpochMs = System.currentTimeMillis();
+        eventRateMeter.mark(1);
+    }
+
+    /**
+     * Gets the last event timestamp Gauge (ms from the Epoch).
+     *
+     * @return the last event timestamp Gauge (ms from the Epoch)
+     */
+    public Gauge<Long> lastEventTimestampGauge() {
+        return lastEventTimestampGauge;
+    }
+
+    /**
+     * Gets the event rate meter.
+     *
+     * @return the event rate meter
+     */
+    public Meter eventRateMeter() {
+        return eventRateMeter;
+    }
+}