Add metrics to SingleThreadedChunker

Using a similar set of metrics to what the `TimedChunker` uses.  It's
helpful for diagnosing queue depth issues.  When we switched to the SPSC
queue, we were surprised the metrics were missing so I figured we could
add them in.

mantis-network/src/main/java/io/reactivex/mantis/network/push/SingleThreadedChunker.java

@@ -16,6 +16,10 @@
 
 package io.reactivex.mantis.network.push;
 
+import io.mantisrx.common.metrics.Counter;
+import io.mantisrx.common.metrics.Metrics;
+import io.mantisrx.common.metrics.MetricsRegistry;
+import io.mantisrx.common.metrics.spectator.MetricGroupId;
 import java.util.ArrayList;
 import java.util.List;
 import java.util.concurrent.Callable;
@@ -33,6 +37,11 @@ public class SingleThreadedChunker<T> implements Callable<Void> {
     int iteration = 0;
     private int index = 0;
 
+    private final Counter numEventsDrained;
+    private final Counter drainTriggeredByTimer;
+    private final Counter drainTriggeredByBatch;
+
+
     public SingleThreadedChunker(ChunkProcessor<T> processor, MonitoredQueue<T> iQ, int chunkSize, long maxChunkInterval, ConnectionManager<T> connMgr) {
         this.inputQueue = iQ;
         this.chunkSize = chunkSize;
@@ -41,6 +50,17 @@ public SingleThreadedChunker(ChunkProcessor<T> processor, MonitoredQueue<T> iQ,
         this.connectionManager = connMgr;
         chunk = new Object[this.chunkSize];
 
+        MetricGroupId metricsGroup = new MetricGroupId("SingleThreadedChunker");
+        Metrics metrics = new Metrics.Builder()
+            .id(metricsGroup)
+            .addCounter("numEventsDrained")
+            .addCounter("drainTriggeredByTimer")
+            .addCounter("drainTriggeredByBatch")
+            .build();
+        numEventsDrained = metrics.getCounter("numEventsDrained");
+        drainTriggeredByTimer = metrics.getCounter("drainTriggeredByTimer");
+        drainTriggeredByBatch = metrics.getCounter("drainTriggeredByBatch");
+        MetricsRegistry.getInstance().registerAndGet(metrics);
     }
 
     private boolean stopCondition() {
@@ -58,6 +78,7 @@ public Void call() throws Exception {
 
                 long currTime = System.currentTimeMillis();
                 if (currTime - maxChunkInterval > chunkStartTime) {
+                    drainTriggeredByTimer.increment();
                     drain();
                 }
                 iteration = 0;
@@ -73,6 +94,7 @@ public Void call() throws Exception {
                     chunk[index++] = ele;
                 }
             } else {
+                drainTriggeredByBatch.increment();
                 drain();
                 chunkStartTime = System.currentTimeMillis();
                 if (stopCondition()) {
@@ -93,6 +115,7 @@ private void drain() {
             }
 
             processor.process(connectionManager, copy);
+            numEventsDrained.increment(copy.size());
             index = 0;
         }
     }