internal/trace: use the correct stack for goroutine naming in v2 traces

Currently goroutine names are determined (for v2 traces) by
internal/tracev/2.Event.Stack, but this is wrong in general. For
example, if we end up seeing a transition from GoNotExist->GoRunnable
(goroutine creation) then we're taking the stack from the creator, not
the created goroutine (which is what we're naming at that point).

Use the StateTransition.Stack instead. This is always the correct one to
use because we're always naming the goroutine that the state transition
is for.

Change-Id: I3fc7c8e4f85dfee3802d666c0c091b6953c7d6cf
Reviewed-on: https://go-review.googlesource.com/c/go/+/544317
Auto-Submit: Michael Knyszek <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>

Author: mknyszek

src/internal/trace/goroutinesv2.go

@@ -318,10 +318,12 @@ func (b *goroutineStatsBuilder) event(ev tracev2.Event) {
 				}
 			}
 
-			// The goroutine hasn't been identified yet. Take any stack we
-			// can get and identify it by the bottom-most frame of that stack.
+			// The goroutine hasn't been identified yet. Take the transition stack
+			// and identify the goroutine by the bottom-most frame of that stack.
+			// This bottom-most frame will be identical for all transitions on this
+			// goroutine, because it represents its immutable start point.
 			if g.PC == 0 {
-				stk := ev.Stack()
+				stk := st.Stack
 				if stk != tracev2.NoStack {
 					var frame tracev2.StackFrame
 					var ok bool