have metadata item fail on create if the key is already present

[danawillow]

Fixes hashicorp/terraform-provider-google#5514.

Also cleaned up tests a bit while I was here.

Release Note Template for Downstream PRs (will be copied)

compute: updated `google_compute_project_metadata_item` to fail on create if its key is already present in the project metadata. 

[modular-magician]

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

Diff report:

Terraform GA: Diff ( 2 files changed, 61 insertions(+), 71 deletions(-))
Terraform Beta: Diff ( 3 files changed, 61 insertions(+), 72 deletions(-))

[modular-magician]

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

Diff report:

Terraform GA: Diff ( 2 files changed, 60 insertions(+), 70 deletions(-))
Terraform Beta: Diff ( 3 files changed, 60 insertions(+), 71 deletions(-))

[rileykarson]

nit: You can "name" the values provided to the function by creating a type instead of using raw bools.

type metadataPresentBehavior bool

const (
	failIfPresent metadataPresentBehavior = true
	expectPresent metadataPresentBehavior = false
)

The behaviour of the function ends up being a little easier to read from the callsite, instead of needing to check the param names.

updateComputeCommonInstanceMetadata(config, projectID, key, &val, int(d.Timeout(schema.TimeoutCreate).Minutes()), failIfPresent)

[modular-magician]

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

Diff report:

Terraform GA: Diff ( 3 files changed, 61 insertions(+), 77 deletions(-))
Terraform Beta: Diff ( 2 files changed, 60 insertions(+), 77 deletions(-))

[modular-magician]

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

Diff report:

Terraform GA: Diff ( 3 files changed, 78 insertions(+), 60 deletions(-))
Terraform Beta: Diff ( 2 files changed, 77 insertions(+), 60 deletions(-))