Fix flush thread waiting time metric when we disable disk index (#3989)

fix flush thread waiting time metric when we disable disk index

Co-authored-by: HaoranYi <[email protected]>

accounts-db/src/bucket_map_holder.rs

@@ -319,9 +319,14 @@ impl<T: IndexValue, U: DiskIndexValue + From<T> + Into<T>> BucketMapHolder<T, U>
         let mut throttling_wait_ms = None;
         loop {
             if !flush {
+                let mut m = Measure::start("wait");
                 self.wait_dirty_or_aged.wait_timeout(Duration::from_millis(
                     self.stats.remaining_until_next_interval(),
                 ));
+                m.stop();
+                self.stats
+                    .bg_waiting_us
+                    .fetch_add(m.as_us(), Ordering::Relaxed);
             } else if self.should_thread_sleep() || throttling_wait_ms.is_some() {
                 let mut wait = std::cmp::min(
                     self.age_timer