Agent Shutdown may cause panic #111
Description
After an agent is shutdown, enqueueStat may cause a nil pointer panic.
https://github.com/pinpoint-apm/pinpoint-go-agent/blob/main/agent.go#L673C1-L683C2
I don't know where exactly calling enqeueStat, but it should be protected to calling a closed agent. (or, ignore the call in enqueueStat)
panic: send on closed channel
goroutine 9347 [running]:
github.com/pinpoint-apm/pinpoint-go-agent.(*agent).enqueueStat(...)
/home1/irteamsu/work/pkg/mod/github.com/pinpoint-apm/[email protected]/agent.go:675
github.com/pinpoint-apm/pinpoint-go-agent.(*agent).sendUrlStatWorker(0xc0002cc640)
/home1/irteamsu/work/pkg/mod/github.com/pinpoint-apm/[email protected]/agent.go:667 +0x1f6
created by github.com/pinpoint-apm/pinpoint-go-agent.(*agent).connectGrpcServer in goroutine 9298
/home1/irteamsu/work/pkg/mod/github.com/pinpoint-apm/[email protected]/agent.go:227 +0x305