Add alarm in callback streaming test

There aren't the right set of alarms in callback streaming reactor, so it exceeds the attempted qps while testing the fixed load streaming scenarios in synthetic benchmarks.

test/cpp/qps/client_callback.cc

@@ -285,8 +285,18 @@ class CallbackStreamingPingPongReactor final
       }
       return;
     }
-    write_time_ = UsageTimer::Now();
-    StartWrite(client_->request());
+    if (!client_->IsClosedLoop()) {
+      gpr_timespec next_issue_time = client_->NextRPCIssueTime();
+      // Start an alarm callback to run the internal callback after
+      // next_issue_time
+      ctx_->alarm_.experimental().Set(next_issue_time, [this](bool ok) {
+        write_time_ = UsageTimer::Now();
+        StartWrite(client_->request());
+      });
+    } else {
+      write_time_ = UsageTimer::Now();
+      StartWrite(client_->request());
+    }
   }
 
   void OnDone(const Status& s) override {