feature: set GOMAXPROCS env var during Set

[xieyuschen]

Currently, when do import _ "go.uber.org/automaxprocs", the GOMAXPROCS is set automatically for users.
But it doesn't set up the GOMAXPROCS environment variable which means if the programs exec some children processes to do some work, the env var needs to be set up manually.

For me, I think there are no side effects to setup the GOMAXPROCS environment variable also when we set up it in the program. So I raised this PR.

Regrads.

[CLAassistant]

All committers have signed the CLA.

[xieyuschen]

Hi @akshayjshah , could you kindly check this PR if you are free?
Regards

[akshayjshah]

Hi! I’m not at Uber any more; you’ll need to have one of the current maintainers review this PR.

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[xieyuschen]

I see, thanks for replying, @akshayjshah

Hi @albertyw @abhinav , could you kindly check it if you have time?

Thanks:)

[albertyw]

I don't think we can merge this. This feature request is essentially the same as the discussion in #29 to expose core count except through an environment variable instead of as part of the library API. See #29 (comment) for more reasoning.

For your use case of setting GOMAXPROCS on children processes, I suggest having your children processes import this library themselves.