Do not pin threads when handling sync metrics

[Floweynt]

The current implementation of Metric#collect for non-async-capable metrics is flawed:

Bukkit.getScheduler().callSyncMethod(plugin, () -> {
    try {
        doCollect();
    }
    catch (Exception e) {
        logException(e);
    }
    return null;
}).get();

get() is implemented in Bukkit with Object#wait. It is not advisable to ever call this method from an Executor, since low level synchronization primitives typically pin the thread. For instance, this will cause a deadlock with CommandAPI, since CommandAPI's reloading mechanism will dispatch some tasks to ForkJoinPool.commonPool(). In the case that all threads are pinned by this plugin, CommandAPI never finishes reloading datapacks and thus blocks the main thread forever (which means callSyncTask's future never completes).

Instead of waiting, we can simply construct a CompletableFuture which is completed whenever the metric is done collecting.