port forward the drive-by fix to measure() from 2466a2f977

pageserver/src/metrics.rs

@@ -1319,12 +1319,12 @@ impl SmgrOpFlushInProgress {
     {
         let mut fut = std::pin::pin!(fut);
 
-        let now = Instant::now();
         // Whenever observe_guard gets called, or dropped,
         // it adds the time elapsed since its last call to metrics.
         // Last call is tracked in `now`.
         let mut observe_guard = scopeguard::guard(
             || {
+                let now = Instant::now();
                 let elapsed = now - self.base;
                 self.global_micros
                     .inc_by(u64::try_from(elapsed.as_micros()).unwrap());