Enhancement

The system variable tidb_enable_gogc_tuner controls to enable or disable GOGC tuner which tune GOGC automaticly. If we enbale GOGC tuner, tidb will call runtime.ReadMemStats to collect memory stats which tiggers STW and blocks the tidb-server instance. We hope that it doesn't call runtime.ReadMemStats if we disable GOGC tuner, but it doesn't, tidb still call runtime.ReadMemStats when GOGC tunner is disabled, we should make it what as we hope.