Add a util method to warm up EventLoopGroup (#3610)

Motivation:

Netty's event loop threads are spawned lazily, i.e. they are created
on the first task is scheduled on. A user sometimes wants to ensure
the event loop threads are started when the server starts up.

Modifications:

- Add `EventLoopGroups.warmUp()`

Result:

- Closes #3363

core/src/main/java/com/linecorp/armeria/common/util/EventLoopGroups.java

@@ -44,6 +44,7 @@
 public final class EventLoopGroups {
 
     private static final EventLoop directEventLoop = new DirectEventLoop();
+    private static final Runnable NO_OP = () -> {};
 
     /**
      * Returns a newly-created {@link EventLoopGroup}.
@@ -108,6 +109,18 @@ public static EventLoopGroup newEventLoopGroup(int numThreads, ThreadFactory thr
         return type.newEventLoopGroup(numThreads, unused -> threadFactory);
     }
 
+    /**
+     * Warms up all {@link EventLoop}s in the given {@code eventLoopGroup}
+     * by making sure all event loop threads are active.
+     */
+    public static EventLoopGroup warmUp(EventLoopGroup eventLoopGroup) {
+        requireNonNull(eventLoopGroup, "eventLoopGroup");
+
+        eventLoopGroup.forEach(executor -> executor.submit(NO_OP));
+
+        return eventLoopGroup;
+    }
+
     /**
      * Returns a special {@link EventLoop} which executes submitted tasks in the caller thread.
      * Note that this {@link EventLoop} will raise an {@link UnsupportedOperationException} for any operations