runtime: factor our oneNewExtraM trace code

runtime: factor our oneNewExtraM trace code

In the interest of further cleaning up the trace.go API, move the trace
logic in oneNewExtraM into its own function.

Change-Id: I5cf478cb8cd0d301ee3b068347ed48ce768b8882
Reviewed-on: https://go-review.googlesource.com/c/go/+/494186
Reviewed-by: Michael Pratt <mpratt@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>