Remove instrumentation from ExecutorService in StreamRouter

The ExecutorService is only used for the TimeLimiter in StreamRouter and the acquired
metrics are basically useless (or derivates of other metrics). Removing the instrumentation
from this particular ExecutorService prevents the instantiation of quite some
InstrumentedRunnables which in turn helps to get some pressure off the GC.

graylog2-server/src/main/java/org/graylog2/streams/StreamRouter.java

@@ -16,7 +16,6 @@
  */
 package org.graylog2.streams;
 
-import com.codahale.metrics.InstrumentedExecutorService;
 import com.codahale.metrics.Meter;
 import com.codahale.metrics.MetricRegistry;
 import com.codahale.metrics.Timer;
@@ -95,10 +94,7 @@ private ExecutorService executorService() {
                 .setNameFormat("stream-router-%d")
                 .setDaemon(true)
                 .build();
-        return new InstrumentedExecutorService(
-                Executors.newCachedThreadPool(threadFactory),
-                metricRegistry,
-                name(this.getClass(), "executor-service"));
+        return Executors.newCachedThreadPool(threadFactory);
     }
 
     private AtomicInteger getFaultCount(String streamId) {