Don't connect in Gitlab::Database.adapter_name

We don't need to connect when requesting the name of the database
adapter. This in turn should prevent us from requesting/leaking
connections just by asking whether we're using PostgreSQL or MySQL.

changelogs/unreleased/fix-ar-connection-leaks.yml

+---
+title: Don't connect in Gitlab::Database.adapter_name
+merge_request:
+author:

lib/gitlab/database.rb

@@ -6,7 +6,7 @@ module Gitlab
     MAX_INT_VALUE = 2147483647
 
     def self.adapter_name
-      connection.adapter_name
+      ActiveRecord::Base.configurations[Rails.env]['adapter']
     end
 
     def self.mysql?

spec/lib/gitlab/database_spec.rb

@@ -5,6 +5,12 @@ class MigrationTest
 end
 
 describe Gitlab::Database, lib: true do
+  describe '.adapter_name' do
+    it 'returns the name of the adapter' do
+      expect(described_class.adapter_name).to be_an_instance_of(String)
+    end
+  end
+
   # These are just simple smoke tests to check if the methods work (regardless
   # of what they may return).
   describe '.mysql?' do