runtime/metrics: update documentation to current interface
The package documentation referenced sample metadata that was removed in CL 282632. Update this documentation to be less specific
about what metadata is available.
Additionally, the documentation on the Sample type referred to Descriptions instead of All as the source of metrics names.
Fixes #44280.
Change-Id: I24fc63a744bf498cb4cd5bda56c1599f6dd75929
Reviewed-on: https://go-review.googlesource.com/c/go/+/292309 Reviewed-by: Michael Knyszek <mknyszek@google.com>
Trust: Michael Knyszek <mknyszek@google.com>
Trust: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>