span.Finish() should be a noop when span is already finished

[tonyo]

At the moment we allow to call Finish() on spans (and, specifically, transactions) multiple times, and run the full Finish() logic every time.

It's not optimal, because finalizing the same span/transaction is not something that we officially support or recommend, and it also might cause a data race, as in the following example:

transaction := StartTransaction(ctx, "op")

go func() {
	for {
		transaction.Finish()
	}
}()

go func() {
	for {
		transaction.Finish()
	}
}()

time.Sleep(100 * time.Second)

Data race log

==================
WARNING: DATA RACE
Read at 0x00c00013c9a8 by goroutine 9:
  github.com/getsentry/sentry-go.(*environmentIntegration).processor()
      /Users/anton/reps/sentry-go/integrations.go:91 +0x3ad
  github.com/getsentry/sentry-go.(*environmentIntegration).processor-fm()
      <autogenerated>:1 +0x4d
  github.com/getsentry/sentry-go.(*Client).prepareEvent()
      /Users/anton/reps/sentry-go/client.go:651 +0xb69
  github.com/getsentry/sentry-go.(*Client).processEvent()
      /Users/anton/reps/sentry-go/client.go:571 +0x4a4
  github.com/getsentry/sentry-go.(*Client).CaptureEvent()
      /Users/anton/reps/sentry-go/client.go:397 +0xa8
  github.com/getsentry/sentry-go.(*Hub).CaptureEvent()
      /Users/anton/reps/sentry-go/hub.go:224 +0x92
  github.com/getsentry/sentry-go.(*Span).Finish()
      /Users/anton/reps/sentry-go/tracing.go:205 +0x2e4
  github.com/getsentry/sentry-go.TestSetContextAndFinishTransaction.func1()
      /Users/anton/reps/sentry-go/tracing_test.go:850 +0x30

Previous write at 0x00c00013c9a8 by goroutine 10:
  github.com/getsentry/sentry-go.(*environmentIntegration).processor()
      /Users/anton/reps/sentry-go/integrations.go:92 +0x3f8
  github.com/getsentry/sentry-go.(*environmentIntegration).processor-fm()
      <autogenerated>:1 +0x4d
  github.com/getsentry/sentry-go.(*Client).prepareEvent()
      /Users/anton/reps/sentry-go/client.go:651 +0xb69
  github.com/getsentry/sentry-go.(*Client).processEvent()
      /Users/anton/reps/sentry-go/client.go:571 +0x4a4
  github.com/getsentry/sentry-go.(*Client).CaptureEvent()
      /Users/anton/reps/sentry-go/client.go:397 +0xa8
  github.com/getsentry/sentry-go.(*Hub).CaptureEvent()
      /Users/anton/reps/sentry-go/hub.go:224 +0x92
  github.com/getsentry/sentry-go.(*Span).Finish()
      /Users/anton/reps/sentry-go/tracing.go:205 +0x2e4
  github.com/getsentry/sentry-go.TestSetContextAndFinishTransaction.func2()
      /Users/anton/reps/sentry-go/tracing_test.go:861 +0x30

Goroutine 9 (running) created at:
  github.com/getsentry/sentry-go.TestSetContextAndFinishTransaction()
      /Users/anton/reps/sentry-go/tracing_test.go:848 +0x199
  testing.tRunner()
      /Users/anton/.asdf/installs/golang/1.20/go/src/testing/testing.go:1576 +0x216
  testing.(*T).Run.func1()
      /Users/anton/.asdf/installs/golang/1.20/go/src/testing/testing.go:1629 +0x47

Goroutine 10 (running) created at:
  github.com/getsentry/sentry-go.TestSetContextAndFinishTransaction()
      /Users/anton/reps/sentry-go/tracing_test.go:859 +0x2b1
  testing.tRunner()
      /Users/anton/.asdf/installs/golang/1.20/go/src/testing/testing.go:1576 +0x216
  testing.(*T).Run.func1()
      /Users/anton/.asdf/installs/golang/1.20/go/src/testing/testing.go:1629 +0x47
==================

The proposal is to make Span.Finish() a no-op after Finish() has already been called once.

Extracted from #570