runtime: MemStats fields should document their runtime/metrics equivalents

[rhysh]

What version of Go are you using (go version)?

go1.21.2

Does this issue reproduce with the latest release?

Yes

What operating system and processor architecture are you using (go env)?

N/A

What did you do?

Considered updating a library to use the new runtime/metrics API, examined https://pkg.go.dev/runtime#MemStats docs to get the details on how to make the move.

What did you expect to see?

References on each field describing which metric names to use with the runtime/metrics package to get the data without a call to runtime.ReadMemStats (and the resulting app-wide stop-the-world pause). A lot of these metrics are pretty subtle, and it takes very careful reading to make sure that the data from the new source will have the same meaning as the data from the old source.

What did you see instead?

The docs for runtime.MemStats and runtime/metrics do not reference each other's fields.

CC @golang/runtime , and @mknyszek in particular