Add context to SessionsController

This doesn't inherit from ApplicationController and so doesn't get a
context by default.

app/controllers/sessions_controller.rb

@@ -26,6 +26,8 @@ class SessionsController < Devise::SessionsController
   before_action :load_recaptcha
   before_action :frontend_tracking_data, only: [:new]
 
+  around_action :set_current_context
+
   after_action :log_failed_login, if: :action_new_and_failed_login?
 
   helper_method :captcha_enabled?, :captcha_on_login_required?
@@ -305,6 +307,13 @@ class SessionsController < Devise::SessionsController
     # We want tracking data pushed to the frontend when the user is _in_ the control group
     frontend_experimentation_tracking_data(:signup_flow, 'start') unless experiment_enabled?(:signup_flow)
   end
+
+  def set_current_context(&block)
+    Gitlab::ApplicationContext.with_context(
+      user: -> { current_user },
+      caller_id: "#{self.class.name}##{action_name}",
+      &block)
+  end
 end
 
 SessionsController.prepend_if_ee('EE::SessionsController')

spec/controllers/sessions_controller_spec.rb

@@ -495,4 +495,45 @@ describe SessionsController do
       expect(session[:failed_login_attempts]).to eq(1)
     end
   end
+
+  describe '#set_current_context' do
+    before do
+      set_devise_mapping(context: @request)
+    end
+
+    context 'when signed in' do
+      let_it_be(:user) { create(:user) }
+
+      before do
+        sign_in(user)
+      end
+
+      it 'sets the username and caller_id in the context' do
+        expect(controller).to receive(:destroy).and_wrap_original do |m, *args|
+          expect(Labkit::Context.current.to_h)
+            .to include('meta.user' => user.username,
+                        'meta.caller_id' => 'SessionsController#destroy')
+
+          m.call(*args)
+        end
+
+        delete :destroy
+      end
+    end
+
+    context 'when not signed in' do
+      it 'sets the caller_id in the context' do
+        expect(controller).to receive(:new).and_wrap_original do |m, *args|
+          expect(Labkit::Context.current.to_h)
+            .to include('meta.caller_id' => 'SessionsController#new')
+          expect(Labkit::Context.current.to_h)
+            .not_to include('meta.user')
+
+          m.call(*args)
+        end
+
+        get :new
+      end
+    end
+  end
 end