Adding a new function to override the client args from terraform provider of synthetics

[AnshitAgrawal]

---

Before the change?

In one of our example, instead of using api..signalfx.com we are making use of external-api..signalfx.com api URL. So in order to achieve it, following support is added in which if apiUrl is passed in providers.tf, it will take precedence otherwise default one will be used.
In order to make that work in synthetics teraform provider, it is required to update the synthetics client code as well so that the non exportable variables can be set from Upstream.

After the change?

This is totally a new function in client code, so it will not affect the existing functionality in any way.

Pull request checklist

• Acceptance Tests have been updated, run (make testacc), and pasted in this PR (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Acceptance Test Output

Does this introduce a breaking change?

• Yes
• No

---

[github-actions]

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[AnshitAgrawal]

I have read the CLA Document and I hereby sign the CLA

[AnshitAgrawal]

Testing results along with changes in splunk/terraform-provider-synthetics#78.

1. Failure logs with worng custom URL -

2025-08-18T19:58:54.916+0530 [ERROR] vertex "synthetics_create_browser_check_v2.browser_v2_foo_check" error: Post "https://api.lab.signalfx.com/v2/synthetics/tests/browser": dial tcp: lookup api.lab.signalfx.com: no such host

╷
│ Error: Post "https://api.lab.signalfx.com/v2/synthetics/tests/browser": dial tcp: lookup api.lab.signalfx.com: no such host
│
│ with synthetics_create_browser_check_v2.browser_v2_foo_check,
│ on a.tf line 17, in resource "synthetics_create_browser_check_v2" "browser_v2_foo_check":
│ 17: resource "synthetics_create_browser_check_v2" "browser_v2_foo_check" {
│
╵
2025-08-18T19:58:54.919+0530 [DEBUG] provider.stdio: received EOF, stopping recv loop: err="rpc error: code = Unavailable desc = error reading from server: EOF"
2025-08-18T19:58:54.920+0530 [DEBUG] provider: plugin process exited: path=/Users/anshiagr/tf-provider/new/terraform-provider-synthetics pid=39882
2025-08-18T19:58:54.921+0530 [DEBUG] provider: plugin exited

2. Logs with no custom URL field [ I abrupty failed as I dont want to apply]

2025-08-18T20:00:25.741+0530 [ERROR] vertex "synthetics_create_browser_check_v2.browser_v2_foo_check" error: Post "https://api.lab0.signalfx.com/v2/synthetics/tests/browser": context deadline exceeded (Client.Timeout exceeded while awaiting headers)
╷
│ Error: Post "https://api.lab0.signalfx.com/v2/synthetics/tests/browser": context deadline exceeded (Client.Timeout exceeded while awaiting headers)
│
│ with synthetics_create_browser_check_v2.browser_v2_foo_check,
│ on a.tf line 16, in resource "synthetics_create_browser_check_v2" "browser_v2_foo_check":
│ 16: resource "synthetics_create_browser_check_v2" "browser_v2_foo_check" {
│
╵
2025-08-18T20:00:25.751+0530 [DEBUG] provider.stdio: received EOF, stopping recv loop: err="rpc error: code = Unavailable desc = error reading from server: EOF"
2025-08-18T20:00:25.760+0530 [DEBUG] provider: plugin process exited: path=/Users/anshiagr/tf-provider/new/terraform-provider-synthetics pid=40106

3. Logs with custom URL and working [ 403 because of VPN issue]

2025-08-18T20:01:20.248+0530 [ERROR] vertex "synthetics_create_browser_check_v2.browser_v2_foo_check" error: unknown error, status code: 403
╷
│ Error: unknown error, status code: 403
│
│ with synthetics_create_browser_check_v2.browser_v2_foo_check,
│ on a.tf line 17, in resource "synthetics_create_browser_check_v2" "browser_v2_foo_check":
│ 17: resource "synthetics_create_browser_check_v2" "browser_v2_foo_check" {
│
╵
2025-08-18T20:01:20.255+0530 [DEBUG] provider.stdio: received EOF, stopping recv loop: err="rpc error: code = Unavailable desc = error reading from server: EOF"
2025-08-18T20:01:20.257+0530 [DEBUG] provider: plugin process exited: path=/Users/anshiagr/tf-provider/new/terraform-provider-synthetics pid=40256
2025-08-18T20:01:20.257+0530 [DEBUG] provider: plugin exited

[AnshitAgrawal]

I have read the Code of Conduct and I hereby accept the Terms