runtime/metrics: expand Read documention with caveats
This change modifies the documentation of Read with some caveats about
reusing the slice passed in to Read as well as with what concurrent
situations are safe.
Change-Id: I76fd31acc67ae384546a8442dfbf9d16b7445cff
Reviewed-on: https://go-review.googlesource.com/c/go/+/275853
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Michael Knyszek <mknyszek@google.com> Reviewed-by: Michael Pratt <mpratt@google.com>