Stop editing the global config object with new useragent entries

[modular-magician]

It looks like some occurrences of this were left in after GoogleCloudPlatform/magic-modules#4016. This means we'd keep appending onto the end of it while a single provider instance was alive, possibly causing issues in large applies.

Also don't append to the user agent if the m.ModuleName value is nil or empty- we would have added a space every time generateUserAgentString was called.

Notably, this breaks module attribution for batched requests. I'm pretty comfortable with that- it just affects service management and IAM, and we can fix them in a followup.

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Release Note Template for Downstream PRs (will be copied)

provider: fixed an issue where the request headers would grow proportionally to the number of resources in a given `terraform apply`

Derived from GoogleCloudPlatform/magic-modules#4123