Only get userAgent when needed

[tyrannosaurus-becks]

The Go SDK is used by Hashicorp for Vault, and it retrieves information from the external environment on package initialization that won't necessarily be needed. This PR moves the call out of package initialization while retaining all normal functionality.

See hashicorp/vault#5324 for more.

[CLAassistant]

All committers have signed the CLA.

[coveralls]

Coverage decreased (-50.7%) to 40.567% when pulling fd883d8 on tyrannosaurus-becks:only-get-system-info-if-needed into 36bf7aa on aliyun:master.

[hangzws]

Thanks for you pr, we will merge it in master later.

[tyrannosaurus-becks]

@hangzws thank you!

Do you currently have any idea of the date on which it will be merged into master? We are deciding whether to use a forked version of this repo for our 0.11.2 release of Vault.

[hangzws]

we will fix it in version 1.9.3

[tyrannosaurus-becks]

@hangzws thank you!