Provide an API to expose the read value without setting GOMAXPROCS

[ajwerner]

The logic to read the core count from cgroups is very valuable. In some settings it might be desirable to read the value but not set GOMAXPROCS with it. This issue requests that maxprocs subpackage provide a method that returns the read value without setting the runtime value.

[prashantv]

Once an API is exposed, it has to remain backwards compatible, so we need to make sure that:

• The API we want to expose belongs in automaxprocs (should it be a separate repository / module?). CGroup parsing is a dependency of automaxprocs, but not necessarily something that has to be coupled to automaxprocs.
• Is the API surface minimized? To avoid breaking version upgrades, we'll need to maintain any APIs exposed, so we need to minimize and ensure that the API surface is correct.
• Can the API evolve to support new requests? E.g., if we needed to support other CGroups resources or versions, can it be come in a clean way.

Given the above, it takes a fair bit of due diligence before we can approve exposing any further APIs.

[ovesh]

Moving CGroup parsing to a separate repo/module sounds like the best path forward IMO.

[zhijian-pro]

I think it could be useful to provide an api to get the cpu num instead of setting it directly. It would be better to let the user set it manually.
We can release a V2 version to make these incompatible changes