Stop editing the global config object with new useragent entries

[rileykarson]

It looks like some occurrences of this were left in after #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`

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 170 files changed, 8 insertions(+), 301 deletions(-))
Terraform Beta: Diff ( 195 files changed, 8 insertions(+), 350 deletions(-))
TF Conversion: Diff ( 1 file changed, 5 insertions(+), 1 deletion(-))

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=152605"

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 170 files changed, 8 insertions(+), 301 deletions(-))
Terraform Beta: Diff ( 195 files changed, 8 insertions(+), 350 deletions(-))
TF Conversion: Diff ( 1 file changed, 5 insertions(+), 1 deletion(-))

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=152613"

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 170 files changed, 8 insertions(+), 301 deletions(-))
Terraform Beta: Diff ( 195 files changed, 8 insertions(+), 350 deletions(-))
TF Conversion: Diff ( 1 file changed, 5 insertions(+), 1 deletion(-))

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=152616"

[modular-magician]

Hi! I'm the modular magician. Your PR generated some diffs in downstreams - here they are.

Diff report:

Terraform GA: Diff ( 173 files changed, 8 insertions(+), 449 deletions(-))
Terraform Beta: Diff ( 198 files changed, 8 insertions(+), 498 deletions(-))
TF Conversion: Diff ( 1 file changed, 5 insertions(+), 1 deletion(-))

[modular-magician]

I have triggered VCR tests based on this PR's diffs. See the results here: "https://ci-oss.hashicorp.engineering/viewQueued.html?itemId=153002"