Reduce poll timer sampling from ~100% to ~10%

This should improve performance just a little bit

src/future.rs

@@ -141,6 +141,9 @@ where
         let out = this.inner.poll(cx);
 
         let elapsed = start.elapsed();
+
+        // only record 1 in 10 polls
+        if elapsed.as_micros() % 10 == 0 {
             if elapsed > Duration::from_micros(10) {
                 metrics::counter!(crate::init_metrics::FUTURE_POLL_TIMER_EXCEEDED, "timer" => this.name.to_string()).increment(1);
                 metrics::histogram!(crate::init_metrics::FUTURE_POLL_TIMER_EXCEEDED_SECONDS, "timer" => this.name.to_string()).record(elapsed.as_secs_f64());
@@ -186,6 +189,7 @@ where
                     elapsed.as_micros()
                 );
             }
+        }
 
         out
     }