chore: remove unnecessary variable (#2400)

Change-Id: Ide37e9226983ffaa32d8d2cda03c1fd8d1ad7fff

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:
- [ ] Make sure to open an issue as a [bug/issue](https://togithub.com/googleapis/java-bigtable/issues/new/choose) before writing your code!  That way we can discuss the change, evaluate designs, and agree on the general idea
- [ ] Ensure the tests and linter pass
- [ ] Code coverage does not decrease (if any source code was changed)
- [ ] Appropriate docs were updated (if necessary)
- [ ] Rollback plan is reviewed and LGTMed
- [ ] All new data plane features have a completed end to end testing plan

Fixes #<issue_number_goes_here> ☕️

If you write sample code, please follow the [samples format](
https://togithub.com/GoogleCloudPlatform/java-docs-samples/blob/main/SAMPLE_FORMAT.md).

google-cloud-bigtable/src/main/java/com/google/cloud/bigtable/data/v2/stub/metrics/BuiltinMetricsTracer.java

@@ -67,7 +67,6 @@ class BuiltinMetricsTracer extends BigtableTracer {
   // Stopwatch is not thread safe so this is a workaround to check if the stopwatch changes is
   // flushed to memory.
   private final Stopwatch serverLatencyTimer = Stopwatch.createUnstarted();
-  private boolean serverLatencyTimerIsRunning = false;
   private final Object timerLock = new Object();
 
   private boolean flowControlIsDisabled = false;
@@ -163,9 +162,8 @@ public void attemptStarted(Object request, int attemptNumber) {
     }
     if (!flowControlIsDisabled) {
       synchronized (timerLock) {
-        if (!serverLatencyTimerIsRunning) {
+        if (!serverLatencyTimer.isRunning()) {
           serverLatencyTimer.start();
-          serverLatencyTimerIsRunning = true;
         }
       }
     }
@@ -198,9 +196,8 @@ public void onRequest(int requestCount) {
       // On request is only called when auto flow control is disabled. When auto flow control is
       // disabled, server latency is measured between onRequest and onResponse.
       synchronized (timerLock) {
-        if (!serverLatencyTimerIsRunning) {
+        if (!serverLatencyTimer.isRunning()) {
           serverLatencyTimer.start();
-          serverLatencyTimerIsRunning = true;
         }
       }
     }
@@ -219,10 +216,9 @@ public void responseReceived() {
     // latency is measured between afterResponse and responseReceived.
     // In all the cases, we want to stop the serverLatencyTimer here.
     synchronized (timerLock) {
-      if (serverLatencyTimerIsRunning) {
+      if (serverLatencyTimer.isRunning()) {
         totalServerLatencyNano.addAndGet(serverLatencyTimer.elapsed(TimeUnit.NANOSECONDS));
         serverLatencyTimer.reset();
-        serverLatencyTimerIsRunning = false;
       }
     }
   }
@@ -235,9 +231,8 @@ public void afterResponse(long applicationLatency) {
       // received. If flow control is disabled but requestLeft is greater than 0,
       // also start the timer to count the time between afterResponse and responseReceived.
       synchronized (timerLock) {
-        if (!serverLatencyTimerIsRunning) {
+        if (!serverLatencyTimer.isRunning()) {
           serverLatencyTimer.start();
-          serverLatencyTimerIsRunning = true;
         }
       }
     }
@@ -324,11 +319,10 @@ private void recordAttemptCompletion(@Nullable Throwable status) {
     // If the attempt failed, the time spent in retry should be counted in application latency.
     // Stop the stopwatch and decrement requestLeft.
     synchronized (timerLock) {
-      if (serverLatencyTimerIsRunning) {
+      if (serverLatencyTimer.isRunning()) {
         requestLeft.decrementAndGet();
         totalServerLatencyNano.addAndGet(serverLatencyTimer.elapsed(TimeUnit.NANOSECONDS));
         serverLatencyTimer.reset();
-        serverLatencyTimerIsRunning = false;
       }
     }