Add a separate grafana.com API URL setting (#59506)

The GrafanaComURL setting is currently used in two places:

- the /api/gnet endpoint, which proxies all requests to the URL
  configured in GrafanaComURL
- OAuth logins using grafana.com, where the auth URL, token URL and
  redirect URL are all configured to use the GrafanaComURL.

This has worked fine until now because almost all Grafana instances have
just used the default value, https://grafana.com. However, we now have a
few different grafana.com's, some of which are behind IAP. The IAP
causes the /api/gnet proxy to fail because the required cookies are not
present in the request (how could they be?). Setting the
[grafana_net.url] setting to an internal-only URL improves the situation
slightly - the proxy works again just fine - but breaks any OAuth logins
using grafana.com, because the user must be redirected to a publicly
accessible URL.

This commit adds an additional setting, `[grafana_com.api_url]`,
which can be used to tell Grafana to use the new API URL when proxying
requests to the grafana.com API, while still using the existing
`GrafanaComURL` setting for other things.

The setting will fall back to the GrafanaComURL setting + "/api" if unset.

pkg/setting/setting.go

@@ -456,8 +456,12 @@ type Cfg struct {
 	// then Live uses AppURL as the only allowed origin.
 	LiveAllowedOrigins []string
 
-	// Grafana.com URL
+	// Grafana.com URL, used for OAuth redirect.
 	GrafanaComURL string
+	// Grafana.com API URL. Can be set separately to GrafanaComURL
+	// in case API is not publicly accessible.
+	// Defaults to GrafanaComURL setting + "/api" if unset.
+	GrafanaComAPIURL string
 
 	// Geomap base layer config
 	GeomapDefaultBaseLayerConfig map[string]interface{}
@@ -1104,6 +1108,8 @@ func (cfg *Cfg) Load(args CommandLineArgs) error {
 	}
 	cfg.GrafanaComURL = GrafanaComUrl
 
+	cfg.GrafanaComAPIURL = valueAsString(iniFile.Section("grafana_com"), "api_url", GrafanaComUrl+"/api")
+
 	imageUploadingSection := iniFile.Section("external_image_storage")
 	cfg.ImageUploadProvider = valueAsString(imageUploadingSection, "provider", "")
 	ImageUploadProvider = cfg.ImageUploadProvider