### 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/main/java/org/elasticsearch/threadpool/ThreadPool.java
* this method, see {@link #relativeTimeInMillis()}.
     */
    public long absoluteTimeInMillis() {
        return cachedTimeThread.absoluteTimeInMillis();
    }
    @Override
    public ThreadPoolInfo info() {
        return threadPoolInfo;
    }
    public Info info(String name) {
        ExecutorHolder holder = executors.get(name);
        if (holder == null) {
            return null;
        }
        return holder.info;
    }
    public ThreadPoolStats stats() {
        List<ThreadPoolStats.Stats> stats = new ArrayList<>();
        for (ExecutorHolder holder : executors.values()) {
            final String name = holder.info.getName();
            
            if ("same".equals(name)) {
                continue;
            }
            int threads = -1;
            int queue = -1;
            int active = -1;
            long rejected = -1;
            int largest = -1;
            long completed = -1;