`static _Bool drop_metric (void)` : I moved the code there.

But why check `drop_metric()` in `plugin_dispatch_values_internal()` ? Why not in ` plugin_dispatch_values()` ?

About the most common condition, I added it, but I'm not sure this is the most common. Most users will have their write queue limit as 0. Well, it not nul, this is the most common condition.