### Description

Today, [force merge is assigned one thread](https://www.elastic.co/guide/en/elasticsearch/reference/current/modules-threadpool.html) only.  

> For [force merge](https://www.elastic.co/guide/en/elasticsearch/reference/current/indices-forcemerge.html) operations. Thread pool type is fixed with a size of 1 and an unbounded queue size.

In a large cluster where there are many processors available, there could be a backlog of force merge queued up.  Perhaps we should look into changing the default of force merge thread_pool to a max of `min(1, (# of allocated processors) / 8)`
server/src/test/java/org/elasticsearch/threadpool/ThreadPoolTests.java
fail();
                }
                assertEquals(threadPool.getThreadContext().getHeader("foo"), "bar");
                assertSame(threadPool.getThreadContext().getTransient("foo"), one);
                assertNull(threadPool.getThreadContext().getHeader("bar"));
                assertNull(threadPool.getThreadContext().getTransient("bar"));
                executed.countDown();
            }, TimeValue.timeValueMillis(randomInt(100)), randomFrom(ThreadPool.Names.SAME, ThreadPool.Names.GENERIC));
            threadPool.getThreadContext().putTransient("bar", "boom");
            threadPool.getThreadContext().putHeader("bar", "boom");
            latch.countDown();
            executed.await();
        } finally {
            latch.countDown();
            terminate(threadPool);
        }
    }
    public void testSchedulerWarnLogging() throws Exception {
        final ThreadPool threadPool = new TestThreadPool(
            "test",
            Settings.builder().put(ThreadPool.SLOW_SCHEDULER_TASK_WARN_THRESHOLD_SETTING.getKey(), "10ms").build()
        );
        final Logger logger = LogManager.getLogger(ThreadPool.class);
        final MockLogAppender appender = new MockLogAppender();
        appender.start();
        try {
            Loggers.addAppender(logger, appender);
            appender.addExpectation(
                new MockLogAppender.SeenEventExpectation(
                    "expected warning for slow task",