Mutable should provide self metrics & decouple with Trigger

[ethe]

In the current, Mutable memtable is declared as:

pub struct Mutable<R>
where
    R: Record,
{
    pub(crate) data: SkipMap<Timestamped<R::Key>, Option<R>>,
    wal: Option<Mutex<WalFile<Box<dyn DynWrite>, R>>>,
    pub(crate) trigger: Arc<Box<dyn Trigger<R> + Send + Sync>>,
}

trigger is initialized in DB and pass into Mutable by Mutable::new, it returns a boolean in each appending to report whether memtable should be froze or not, and this boolean bubbles again up to the top-level (DB or Transaction) . The triggering path is really long and not efficient. Mutable should give a method to report its metrics,Triggerer is able to observe this metrics and decide the freezing timing, it is a possible efficient way to make the freezing trigger clear.