internal/trace: use RFC3339Nano for wall clock snapshots in Event.String

Improve the quality of life for people who use go tool trace -d=parsed
to look at clock snapshot wall timestamps. Many use cases will benefit
from seing the timestamp in sub-second resolution.

Change-Id: I6a6a696403a2164db0c12789c764e22a5c519b1c
Reviewed-on: https://go-review.googlesource.com/c/go/+/697355
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

src/internal/trace/event.go

@@ -879,7 +879,7 @@ func (e Event) String() string {
 			fmt.Fprintf(&sb, " Trace=%d Mono=%d Wall=%s",
 				s.ClockSnapshot.Trace,
 				s.ClockSnapshot.Mono,
-				s.ClockSnapshot.Wall.Format(time.RFC3339),
+				s.ClockSnapshot.Wall.Format(time.RFC3339Nano),
 			)
 		}
 	}