If sc.Telemetry.Disabled=true, trace.StopAgent will panic.

[ansoda]

Describe the bug
A clear and concise description of what the bug is.

To Reproduce
Steps to reproduce the behavior, if applicable:

1. The code is

   func (sc ServiceConf) SetUp() error {
    if len(sc.Log.ServiceName) == 0 {
    	sc.Log.ServiceName = sc.Name
    }
    if err := logx.SetUp(sc.Log); err != nil {
    	return err
    }
   
    sc.initMode()
    prometheus.StartAgent(sc.Prometheus)
   
    if len(sc.Telemetry.Name) == 0 {
    	sc.Telemetry.Name = sc.Name
    }
    trace.StartAgent(sc.Telemetry)
    proc.AddShutdownListener(func() {
    	trace.StopAgent()
    })
   
    if len(sc.MetricsUrl) > 0 {
    	stat.SetReportWriter(stat.NewRemoteWriter(sc.MetricsUrl))
    }
    devserver.StartAgent(sc.DevServer)
   
    return nil
   }

2. The error is

If sc.Telemetry.Disabled=true, trace.StopAgent will panic.

[chenquan]

Can you submit a PR?