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>