doc/next: release note for scheduler metrics

For #15490.

Change-Id: Ic268a7d1d54814e903fc44f5f143008fb5d6dcde
Reviewed-on: https://go-review.googlesource.com/c/go/+/725663
TryBot-Bypass: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>

diff --git a/doc/next/6-stdlib/99-minor/runtime/metrics/15490.md b/doc/next/6-stdlib/99-minor/runtime/metrics/15490.md
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+Several new scheduler metrics have been added, including counts of
+goroutines in various states (waiting, runnable, etc.) under the
+`/sched/goroutines` prefix, the number of OS threads the runtime is
+aware of with `/sched/threads:threads`, and the total number of
+goroutines created by the program with
+`/sched/goroutines-created:goroutines`.