runtime: move goroutine stack exists events after span events

runtime: move goroutine stack exists events after span events

Preserving an ordering wherein spans are always named before their
constituents makes parsing much more straightforward.

Change-Id: Ida8764801a6c516d672ba343e4ce01b66ce9b427
Reviewed-on: https://go-review.googlesource.com/c/go/+/586999
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>