Dont use DB connection in GlobalSearchSampler

As discovered in https://gitlab.com/gitlab-org/gitlab/-/issues/231248
the `GlobalSearchSampler` which invokes this
`Elastic::MetricsUpdateService` is using an extra DB connection since
it runs in it's own thread. This extra DB connection is not accounted
for and can cause issues with the connection pool.

We could account for this extra connection but it turns out the 2
database queries happening in here aren't really necessary anyway. The
first checks if the Elasticsearch integration is enabled. Skipping this
is safe since it will just set `0` for these metrics which is not a
problem. The 2nd checks to see if prometheus monitoring is enabled. But
this check was already happening before spawning the
`GlobalSearchSampler` in
[`config/initializers/7_prometheus_metrics.rb`](
https://gitlab.com/gitlab-org/gitlab/-/blob/17ab3d1634f5f1ca82334f270cf8239d7170b0a2/config/initializers/7_prometheus_metrics.rb#L41
) so we can safely skip that too.

The only other things happening now in this class are requests to Redis
and don't require a DB connection.

ee/changelogs/unreleased/231248-dont-use-db-in-global-search-sampler.yml

+---
+title: Dont use DB connection in GlobalSearchSampler
+merge_request: 38138
+author:
+type: fixed

ee/lib/elastic/metrics_update_service.rb

@@ -3,9 +3,6 @@
 module Elastic
   class MetricsUpdateService
     def execute
-      return unless elasticsearch_enabled?
-      return unless prometheus_enabled?
-
       incremental_gauge = Gitlab::Metrics.gauge(:global_search_bulk_cron_queue_size, 'Number of incremental database updates waiting to be synchronized to Elasticsearch', {}, :max)
       incremental_gauge.set({}, Elastic::ProcessBookkeepingService.queue_size)
 
@@ -15,15 +12,5 @@ module Elastic
       awaiting_indexing_gauge = Gitlab::Metrics.gauge(:global_search_awaiting_indexing_queue_size, 'Number of database updates waiting to be synchronized to Elasticsearch while indexing is paused.', {}, :max)
       awaiting_indexing_gauge.set({}, Elastic::IndexingControlService.queue_size)
     end
-
-    private
-
-    def elasticsearch_enabled?
-      ::Gitlab::CurrentSettings.elasticsearch_indexing?
-    end
-
-    def prometheus_enabled?
-      ::Gitlab::Metrics.prometheus_metrics_enabled?
-    end
   end
 end

ee/spec/services/elastic/metrics_update_service_spec.rb

@@ -5,11 +5,6 @@ require 'spec_helper'
 RSpec.describe Elastic::MetricsUpdateService, :prometheus do
   subject { described_class.new }
 
-  before do
-    stub_ee_application_setting(elasticsearch_indexing: true)
-    allow(Gitlab::Metrics).to receive(:prometheus_metrics_enabled?).and_return(true)
-  end
-
   describe '#execute' do
     it 'sets gauges' do
       expect(Elastic::ProcessBookkeepingService).to receive(:queue_size).and_return(4)
@@ -37,29 +32,5 @@ RSpec.describe Elastic::MetricsUpdateService, :prometheus do
 
       subject.execute
     end
-
-    context 'when prometheus metrics is disabled' do
-      before do
-        allow(Gitlab::Metrics).to receive(:prometheus_metrics_enabled?).and_return(false)
-      end
-
-      it 'does not set a gauge' do
-        expect(Gitlab::Metrics).not_to receive(:gauge)
-
-        subject.execute
-      end
-    end
-
-    context 'when elasticsearch indexing and search is disabled' do
-      before do
-        stub_ee_application_setting(elasticsearch_indexing: false)
-      end
-
-      it 'does not set a gauge' do
-        expect(Gitlab::Metrics).not_to receive(:gauge)
-
-        subject.execute
-      end
-    end
   end
 end