Make it possible to edit Geo primary through API

Upon a customer request make it possible to edit the Geo primary node
through the API.

Technically there is no reason to disallow it, since admins are also
able to modify nodes on the Admin panel.

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/9454

doc/api/geo_nodes.md

@@ -70,7 +70,7 @@ Example response:
 
 ## Edit a Geo node
 
-Updates an existing Geo secondary node. The primary node cannot be edited.
+Updates settings of an existing Geo node.
 
 _This can only be run against a primary Geo node._
 
@@ -92,8 +92,8 @@ Example response:
 ```json
 {
   "id": 1,
-  "url": "https://primary.example.com/",
-  "primary": true,
+  "url": "https://secondary.example.com/",
+  "primary": false,
   "enabled": true,
   "current": true,
   "files_max_capacity": 10,

ee/changelogs/unreleased/tc-geo-open-edit-primary-api.yml

+---
+title: Make it possible to edit Geo primary through API
+merge_request: 9328
+author:
+type: changed

ee/lib/api/geo_nodes.rb

@@ -138,7 +138,7 @@ module API
         #
         # Example request:
         #   PUT /geo_nodes/:id
-        desc 'Edit an existing Geo secondary node' do
+        desc 'Update an existing Geo node' do
           success EE::API::Entities::GeoNode
         end
         params do
@@ -153,9 +153,7 @@ module API
 
           update_params = declared_params(include_missing: false)
 
-          if geo_node.primary?
-            forbidden!('Primary node cannot be edited')
-          elsif geo_node.update(update_params)
+          if geo_node.update(update_params)
             present geo_node, with: EE::API::Entities::GeoNode
           else
             render_validation_error!(geo_node)

ee/spec/requests/api/geo_nodes_spec.rb

@@ -223,6 +223,18 @@ describe API::GeoNodes, :geo, :prometheus, api: true do
       expect(response).to match_response_schema('public_api/v4/geo_node', dir: 'ee')
       expect(json_response).to include(params)
     end
+
+    it 'can update primary' do
+      params = {
+        url: 'https://updated.example.com/'
+      }.stringify_keys
+
+      put api("/geo_nodes/#{primary.id}", admin), params: params
+
+      expect(response).to have_gitlab_http_status(200)
+      expect(response).to match_response_schema('public_api/v4/geo_node', dir: 'ee')
+      expect(json_response).to include(params)
+    end
   end
 
   describe 'DELETE /geo_nodes/:id' do