Fix mysql2 query size

db/migrate/20180831164904_fix_prometheus_metric_query_limits.rb

+# frozen_string_literal: true
+
+# See http://doc.gitlab.com/ce/development/migration_style_guide.html
+# for more information on how to write migrations for GitLab.
+require Rails.root.join('db/migrate/prometheus_metrics_limits_to_mysql')
+
+class FixPrometheusMetricQueryLimits < ActiveRecord::Migration
+  include Gitlab::Database::MigrationHelpers
+
+  def up
+    PrometheusMetricsLimitsToMysql.new.up
+  end
+
+  def down
+    # no-op
+  end
+end

db/migrate/prometheus_metrics_limits_to_mysql.rb

+class PrometheusMetricsLimitsToMysql < ActiveRecord::Migration
+  DOWNTIME = false
+
+  def up
+    return unless Gitlab::Database.mysql?
+
+    change_column :prometheus_metrics, :query, :text, limit: 4096, default: nil
+  end
+
+  def down
+  end
+end