switch to microseconds for remote execution metric (#11464)

Switch to microseconds for the metric for remote execution time to first response. This will provide better resolution, and should be switched now before this metric is ever documented or exposed anywhere.

src/rust/engine/process_execution/src/remote.rs

@@ -294,15 +294,14 @@ impl CommandRunner {
       let item = stream.next().await;
 
       if let Some(start_time) = start_time_opt.take() {
-        let timing: Result<i64, _> = Instant::now()
+        let timing: Result<u64, _> = Instant::now()
           .duration_since(start_time)
-          .as_millis()
+          .as_micros()
           .try_into();
         if let Ok(obs) = timing {
-          context.workunit_store.record_observation(
-            ObservationMetric::RemoteExecutionRPCFirstResponseTime,
-            obs as u64,
-          );
+          context
+            .workunit_store
+            .record_observation(ObservationMetric::RemoteExecutionRPCFirstResponseTime, obs);
         }
       }