Implement `/hooks/:id` GET API for system hooks

Closes: https://gitlab.com/gitlab-org/gitlab/-/issues/353819

doc/api/system_hooks.md

@@ -46,6 +46,43 @@ Example response:
 ]
 ```
 
+## Get system hook
+
+> [Introduced](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/81595) in GitLab 14.9.
+
+Get a system hook by its ID.
+
+```plaintext
+GET /hooks/:id
+```
+
+| Attribute | Type | Required | Description |
+| --------- | ---- | -------- | ----------- |
+| `id` | integer | yes | The ID of the hook |
+
+Example request:
+
+```shell
+curl --header "PRIVATE-TOKEN: <your_access_token>" "https://gitlab.example.com/api/v4/hooks/1"
+```
+
+Example response:
+
+```json
+[
+  {
+    "id": 1,
+    "url": "https://gitlab.example.com/hook",
+    "created_at": "2016-10-31T12:32:15.192Z",
+    "push_events": true,
+    "tag_push_events": false,
+    "merge_requests_events": true,
+    "repository_update_events": true,
+    "enable_ssl_verification": true
+  }
+]
+```
+
 ## Add new system hook
 
 Add a new system hook.

lib/api/system_hooks.rb

@@ -22,6 +22,18 @@ module API
         present paginate(SystemHook.all), with: Entities::Hook
       end
 
+      desc 'Get a hook' do
+        success Entities::Hook
+      end
+      params do
+        requires :id, type: Integer, desc: 'The ID of the system hook'
+      end
+      get ":id" do
+        hook = SystemHook.find(params[:id])
+
+        present hook, with: Entities::Hook
+      end
+
       desc 'Create a new system hook' do
         success Entities::Hook
       end

spec/requests/api/system_hooks_spec.rb

@@ -48,6 +48,48 @@ RSpec.describe API::SystemHooks do
     end
   end
 
+  describe "GET /hooks/:id" do
+    context "when no user" do
+      it "returns authentication error" do
+        get api("/hooks/#{hook.id}")
+
+        expect(response).to have_gitlab_http_status(:unauthorized)
+      end
+    end
+
+    context "when not an admin" do
+      it "returns forbidden error" do
+        get api("/hooks/#{hook.id}", user)
+
+        expect(response).to have_gitlab_http_status(:forbidden)
+      end
+    end
+
+    context "when authenticated as admin" do
+      it "gets a hook", :aggregate_failures do
+        get api("/hooks/#{hook.id}", admin)
+
+        expect(response).to have_gitlab_http_status(:ok)
+        expect(json_response).to match(
+          'id' => be(hook.id),
+          'url' => eq(hook.url),
+          'created_at' => eq(hook.created_at.iso8601(3)),
+          'push_events' => be(hook.push_events),
+          'tag_push_events' => be(hook.tag_push_events),
+          'merge_requests_events' => be(hook.merge_requests_events),
+          'repository_update_events' => be(hook.repository_update_events),
+          'enable_ssl_verification' => be(hook.enable_ssl_verification)
+        )
+      end
+
+      it 'returns 404 if the system hook does not exist' do
+        get api("/hooks/#{non_existing_record_id}", admin)
+
+        expect(response).to have_gitlab_http_status(:not_found)
+      end
+    end
+  end
+
   describe "POST /hooks" do
     it "creates new hook" do
       expect do