Make fixture message more descriptive

+ use strip_heredoc to make the text in tests much more readable

db/fixtures/production/010_settings.rb

@@ -24,6 +24,6 @@ if envs.any? {|env_name| ENV[env_name].present? }
   if ENV['GITLAB_PROMETHEUS_METRICS_ENABLED'].present?
     value = Gitlab::Utils.to_boolean(ENV['GITLAB_PROMETHEUS_METRICS_ENABLED'])
     settings.prometheus_metrics_enabled = value
-    save(settings, 'GITLAB_PROMETHEUS_METRICS_ENABLED')
+    save(settings, 'Prometheus metrics enabled flag')
   end
 end

spec/lib/gitlab/health_checks/prometheus_text_format_spec.rb

@@ -11,12 +11,13 @@ describe Gitlab::HealthChecks::PrometheusTextFormat do
     end
 
     it 'marshal to text with non repeating type definition' do
-      expected = <<-EXPECTED
-# TYPE metric1 gauge
-metric1 1
-# TYPE metric2 gauge
-metric2 2
-EXPECTED
+      expected = <<-EXPECTED.strip_heredoc
+        # TYPE metric1 gauge
+        metric1 1
+        # TYPE metric2 gauge
+        metric2 2
+      EXPECTED
+
       expect(subject.marshal(sample_metrics)).to eq(expected.chomp)
     end
 
@@ -30,12 +31,12 @@ EXPECTED
       end
 
       it 'marshal to text with non repeating type definition' do
-        expected = <<-EXPECTED
-# TYPE metric1 gauge
-metric1 1
-metric1 2
-# TYPE metric2 gauge
-metric2 3
+        expected = <<-EXPECTED.strip_heredoc
+          # TYPE metric1 gauge
+          metric1 1
+          metric1 2
+          # TYPE metric2 gauge
+          metric2 3
         EXPECTED
         expect(subject.marshal(sample_metrics)).to eq(expected.chomp)
       end