Do not run service discovery for DB load balancing for rake tasks

What does this MR do and why?

Do not run service discovery for DB load balancing for rake tasks

It turns out we don't want to setup DB load balancing for rake task. This is implemented correctly for the host based load balancing code, but not implemented correctly for the service discovery based load balancing code.

Make this two paths consistent with each other.

Related issue: #346765 (closed)

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How to set up and validate locally

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1. Setup config/database.yml to add the following lines (reference https://docs.gitlab.com/ee/administration/postgresql/database_load_balancing.html#service-discovery). You can use the value s below as is

   load_balancing:
     discover:
       nameserver: 'localhost'
       record: something.localhost

2. Apply the following patch. It is necessary to mimic that we in a Rake task, and that we have a response from the service discovery DNS query. (Or you can setup a real DNS server, and run a rake task)

diff --git a/lib/gitlab/database/load_balancing/configuration.rb b/lib/gitlab/database/load_balancing/configuration.rb
index da313361073..3c475a4a891 100644
--- a/lib/gitlab/database/load_balancing/configuration.rb
+++ b/lib/gitlab/database/load_balancing/configuration.rb
@@ -102,9 +102,10 @@ def pool_size
         # This is disabled for Rake tasks to ensure e.g. database migrations
         # always produce consistent results.
         def load_balancing_enabled?
-          return false if Gitlab::Runtime.rake?
+          return false
+          #return false if Gitlab::Runtime.rake?
 
-          hosts.any? || service_discovery_enabled?
+          #hosts.any? || service_discovery_enabled?
         end
 
         def service_discovery_enabled?
diff --git a/lib/gitlab/database/load_balancing/service_discovery.rb b/lib/gitlab/database/load_balancing/service_discovery.rb
index dfd4892371c..23442b74fe3 100644
--- a/lib/gitlab/database/load_balancing/service_discovery.rb
+++ b/lib/gitlab/database/load_balancing/service_discovery.rb
@@ -109,7 +109,8 @@ def perform_service_discovery
         # The return value is the amount of time (in seconds) to wait before
         # checking the DNS record for any changes.
         def refresh_if_necessary
-          interval, from_dns = addresses_from_dns
+          #interval, from_dns = addresses_from_dns
+          interval, from_dns = 10, [Address.new('127.0.0.1', 8022)]
 
           current = addresses_from_load_balancer
 

3. Run a rails console. For master, you will see the following errors in log/exceptions_json.log. For this branch, there will be no errors

{"severity":"ERROR","time":"2021-12-01T08:04:03.711Z","correlation_id":null,"exception.class":"NoMethodError","exception.message":"undefined method `host' for #\u003cGitlab::Database::LoadBalancing::PrimaryHost:0x00007f9d89e5b978\u003e","exception.backtrace":[],"user.username":null,"tags.program":"console","tags.locale":"en","tags.feature_category":null,"tags.correlation_id":null}

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