Allow adding headroom to the connection pool

This allows setting the `DB_POOL_HEADROOM` env variable to add a
number of connections to the pool.

In normal situations, these connections should not be used, but it
allows us to investigate the problem without degrading the service.

config/initializers/database_config.rb

@@ -30,7 +30,7 @@ if Gitlab::Runtime.multi_threaded?
       Rails.application.config.database_configuration[Rails.env]
   previous_db_pool_size = db_config['pool']
 
-  db_config['pool'] = [db_config['pool'].to_i, max_threads].max
+  db_config['pool'] = [db_config['pool'].to_i, max_threads].max + ENV["DB_POOL_HEADROOM"].to_i
 
   ActiveRecord::Base.establish_connection(db_config)
 

spec/initializers/database_config_spec.rb

@@ -48,6 +48,21 @@ describe 'Database config initializer' do
         expect { subject }.not_to change { Gitlab::Database.config['pool'] }
       end
     end
+
+    context "when specifying headroom through an ENV variable" do
+      let(:headroom) { 10 }
+
+      before do
+        stub_database_config(pool_size: 1)
+        stub_env("DB_POOL_HEADROOM", headroom)
+      end
+
+      it "adds headroom on top of the calculated size" do
+        expect { subject }.to change { Gitlab::Database.config['pool'] }
+                                .from(1)
+                                .to(max_threads + headroom)
+      end
+    end
   end
 
   context "when using single-threaded runtime" do