Accept custom transport of custom http client

[jkroepke]

Hi,

I'm looking into setting up the discovery integration for STACKIT in prometheus/prometheus.

While the integration generally works fine, I encountered some issues injecting the http.RoundTripper from the Prometheus ecosystem into the SDK CLI.

Ref: https://github.com/jkroepke/prometheus/blob/8bec5e7cce575b18af42fe8cd67ee0fd5f47ac73/discovery/stackit/server.go#L91

It seems that each authentication method creates a new http.Client instead of simply extending the chain of RoundTripper functions.

This PR modifies the behavior so that the auth function accepts an existing http.RoundTripper and extends it, as is usual in the Go ecosystem.

Theoretically, this avoids the need for proprietary middleware approaches, as existing middleware can be injected through the Transport of a custom http.Client. That's how it usually works in the Go ecosystem.

[bahkauv70]

hello @jkroepke , thanks for your contribution! I think this would be a good idea, but the current PR breaks backwards compatibility. In the present form, all projects using the SDK would have to be adapted to the changed api. In order to go forward with this, the compatibility to existing projects would have to be maintained

[jkroepke]

Hi @bahkauv70

the current approach should preserve the backwards compatibility.

[marceljk]

Hi @jkroepke,

Thanks for your contribution! The PR looks good. Only a nitpick, can you add some changelogs to these two files:

https://github.com/stackitcloud/stackit-sdk-go/blob/main/core/CHANGELOG.md
https://github.com/stackitcloud/stackit-sdk-go/blob/main/CHANGELOG.md

[jkroepke]

Hi,

I added some test case to increase the test coverage. At least with that tests, I feel a bit more confident that everything still works.

[marceljk]

Hi,

there are some linter issues which you can check here in the pipeline or you can run make lint to see them local

[marceljk]

Thanks for your contribution 😃

[henninge]

Hi!
Thanks for adding this. I stumbled onto this when trying to use the HTTPClient from from golang.org/x/oauth2/clientcredentials.

First I tried this which did not work (no credentials found):

client, err := dns.NewAPIClient(config.WithHTTPClient(httpClient))

My initial workaround was:

client, err := dns.NewAPIClient(config.WithHTTPClient(httpClient), config.WithCustomAuth(httpClient.Transport))

With this PR, though, using the existing transport from the httpClient is put in the NoAuth branch of execution, so I have to do this, to use is.

client, err := dns.NewAPIClient(config.WithHTTPClient(httpClient), config.WithoutAuthentication())

Is this the intended way to use this? The WithoutAuthentication looks rather counter-intuitive.

The docstring for WithHTTPClient states "Warning: providing this option overrides authentication, if you just want to customize the http client parameters except Transport ..." which makes it sound that the transport provided with the HTTPClient takes precedence over any other configuration. This is also what I would expect.

If this is still a bug and not just a misunderstanding on my part, I am happy to provide a PR.