Merge branch 'db/prometheus-metrics-not-null-constraints' into 'master'

Add not null constraints to prometheus_metrics table columns

Closes #66663

See merge request gitlab-org/gitlab-ce!32339

db/migrate/20190828110802_add_not_null_constraints_to_prometheus_metrics_y_label_and_unit.rb

+class AddNotNullConstraintsToPrometheusMetricsYLabelAndUnit < ActiveRecord::Migration[5.2]
+  DOWNTIME = false
+
+  def change
+    change_column_null(:prometheus_metrics, :y_label, false)
+    change_column_null(:prometheus_metrics, :unit, false)
+  end
+end

db/schema.rb

@@ -2869,8 +2869,8 @@ ActiveRecord::Schema.define(version: 2019_09_02_131045) do
     t.integer "project_id"
     t.string "title", null: false
     t.string "query", null: false
-    t.string "y_label"
-    t.string "unit"
+    t.string "y_label", null: false
+    t.string "unit", null: false
     t.string "legend"
     t.integer "group", null: false
     t.datetime_with_timezone "created_at", null: false