Support for custom (enterprise) domains in `site` configuration variable

[inhearst]

Is your feature request related to a problem? Please describe.
I use Datadog under an enterprise domain (like xxx.datadoghq.com) but it does not seem like I can set the URL in the configuration to this. I am able to use the Ruby API with this URL and it works correctly. Same as #1913 but it was closed for some reason.

Describe the solution you'd like
Ability to set the site and similar variables to our custom domain.

Describe alternatives you've considered
I can use another language, however a package I want to use (DD query parser) is only available in golang.

Additional context
Error info:

% go run main.go
Error when calling `MetricsApi.ListMetricAssets`: the variable site in the server URL has invalid value xxx.datadoghq.com. Must be [datadoghq.com us3.datadoghq.com us5.datadoghq.com ap1.datadoghq.com datadoghq.eu ddog-gov.com]
Full HTTP response: <nil>
Response from `MetricsApi.ListMetricAssets`:
{}

[therve]

Hi,

Note that you should be able to use regular domain names for your organization, you don't have to use the custom. If you do, you need to set the ContextServerIndex context variable:

	ctx := datadog.NewDefaultContext(context.Background())
	ctx = context.WithValue(ctx, datadog.ContextServerIndex, 1)
	ctx = context.WithValue(ctx, datadog.ContextServerVariables,
		map[string]string{"name": "abc.datadoghq.com"},
	)

Let me know if that works for you.

[github-actions]

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