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.

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Generated Terraform, and ran make test and make lint to ensure it passes unit and linter tests.
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Ran relevant acceptance tests (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
• Read the Release Notes Guide before writing my release note below.

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

[ghost]

I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues.

If you feel this issue should be reopened, we encourage creating a new issue linking back to this one for added context. If you feel I made an error 🤖 🙉 , please reach out to my human friends 👉 [email protected]. Thanks!