Fix DB connection check for Geo user routing

Overriding Geo routes should happen if the current node is a secondary.

However, simply checking `connected?` on the GeoNode model won't work
because Rails releases the DB connection before reloading routes
in the initializing phase, resulting in the GeoNode#connected? method
being an unreliable indicator for the DB connectivity.

Changelog: fixed
EE: true

ee/lib/gitlab/geo.rb

@@ -44,7 +44,11 @@ module Gitlab
     end
 
     def self.connected?
-      GeoNode.connected? && GeoNode.table_exists?
+      # GeoNode#connected? only attempts to use existing DB connections so it can't
+      # be relied upon in initializers, without this active DB connectivity check.
+      active_db_connection = GeoNode.connection_pool.with_connection(&:active?) rescue false
+
+      active_db_connection && GeoNode.table_exists?
     end
 
     def self.enabled?

ee/spec/lib/gitlab/geo_spec.rb

@@ -165,8 +165,8 @@ RSpec.describe Gitlab::Geo, :geo, :request_store do
 
   describe '.connected?' do
     context 'when there is a database issue' do
-      it 'returns false when database connection is down' do
-        allow(GeoNode).to receive(:connected?) { false }
+      it 'returns false when it cannot open an active database connection' do
+        allow(GeoNode.connection).to receive(:active?).and_return(false)
 
         expect(described_class.connected?).to be_falsey
       end