What does time specified in a stack trace mean?

I'm reading stack trace of a go program for debugging.

goroutine 6 [chan receive]:
go.elastic.co/apm.(*Tracer).loop.func2(0xc000042420, 0xc0000423c0, 0x1551980, 0xc0004da100, 0xc000074300, 0xc000001080, 0xc00000e018)
	/app/vendor/go.elastic.co/apm/tracer.go:803 +0x21a
created by go.elastic.co/apm.(*Tracer).loop
	/app/vendor/go.elastic.co/apm/tracer.go:800 +0x36e

goroutine 8 [chan receive, 21 minutes]:
github.com/getsentry/sentry-go.(*HTTPTransport).worker(0xc0003025b0)
	/app/vendor/github.com/getsentry/sentry-go/transport.go:387 +0x77
created by github.com/getsentry/sentry-go.(*HTTPTransport).Configure.func1
	/app/vendor/github.com/getsentry/sentry-go/transport.go:255 +0x3e

What does the time 21 minutes signify in the second go routine? This time is absent from the first one. I looked up in the documentation but couldn't find what this time is for.

It's the approximate amount of time the goroutine was blocked. It is only displayed if the amount of time the goroutine was blocked is at least 1 minute.

You can look at the code that produces this output in the go runtime source, runtime/traceback.go (linked is the go1.16.6 version of the code).