Do not run telemetry loops when it's disabled in config

Telemetry can not be enabled by global variable when it's disabled in
config file. Therefore we don't need to run telemetry related loops at
all when telemetry is disable from global config.

Signed-off-by: Xiaoguang Sun <sunxiaoguang@gmail.com>

session/session.go

@@ -3373,10 +3373,14 @@ func BootstrapSession(store kv.Storage) (*domain.Domain, error) {
 	if dom.GetEtcdClient() != nil {
 		// We only want telemetry data in production-like clusters. When TiDB is deployed over other engines,
 		// for example, unistore engine (used for local tests), we just skip it. Its etcd client is nil.
-		go func() {
-			dom.TelemetryReportLoop(ses[5])
-			dom.TelemetryRotateSubWindowLoop(ses[5])
-		}()
+		if config.GetGlobalConfig().EnableTelemetry {
+			// There is no way to turn telemetry on with global variable `tidb_enable_telemetry`
+			// when it is disabled in config. See IsTelemetryEnabled function in telemetry/telemetry.go
+			go func() {
+				dom.TelemetryReportLoop(ses[5])
+				dom.TelemetryRotateSubWindowLoop(ses[5])
+			}()
+		}
 	}
 
 	planReplayerWorkerCnt := config.GetGlobalConfig().Performance.PlanReplayerDumpWorkerConcurrency