Set a name to threads created by Scheduler Threadfactory (#1307)

Use prefix "prometheus-metrics-scheduler-" plus counter.

Used in conjunction with
https://issues.apache.org/jira/browse/KAFKA-18982

---------

Signed-off-by: Edoardo Comar <[email protected]>

Author: edoardocomar

prometheus-metrics-core/src/main/java/io/prometheus/metrics/core/util/Scheduler.java

@@ -14,9 +14,15 @@
 public class Scheduler {
 
   private static class DaemonThreadFactory implements ThreadFactory {
+    private static int threadNum;
+
+    private static synchronized int nextThreadNum() {
+      return threadNum++;
+    }
+
     @Override
     public Thread newThread(Runnable runnable) {
-      Thread thread = new Thread(runnable);
+      Thread thread = new Thread(runnable, "prometheus-metrics-scheduler-" + nextThreadNum());
       thread.setDaemon(true);
       return thread;
     }