proposal: runtime: deprecate MemStats.Lookups, possibly retroactively as of Go 1.11

[mknyszek]

This MemStats field has been set to zero since the Go 1.11 release (41e6abd), or for about 5 years. However, this isn't documented.

Given that we now have the runtime/metrics package, I don't see a reason to try and define the field as anything other than "this value is always zero." Personally, I think we should mention this in the documentation just to be honest about what the state of the world is, and has been since Go 1.11. It certainly stopped working entirely in that release, and we have no plans to change that.

However, I am unsure what the best way to move forward is mechanically, hence I bring it to the proposal process which handles API deprecation.

Should this be formally retroactively deprecated in Go 1.11? (i.e. all supported releases have their api/go1.11.txt updated) Or is it policy to not change history in this way, and instead just deprecate it in the next release?

I don't really care either way, but I feel quite comfortable with documenting this field as zero going forward in perpetuity.