Add index to events table support UMAU metric

One of the UMAU supporting metrics from Service Ping payalod
started to fail, due to timeout starting point batching query based on
created_at and id attribute. In order to fix this metric new index is
needed on events table.

Changelog: performance

db/post_migrate/20211028123412_add_async_index_on_events_using_btree_created_at_id.rb

+# frozen_string_literal: true
+
+class AddAsyncIndexOnEventsUsingBtreeCreatedAtId < Gitlab::Database::Migration[1.0]
+  INDEX_NAME = 'index_events_on_created_at_and_id'
+  TABLE = :events
+  COLUMNS = %i[created_at id]
+  CONSTRAINTS = "created_at > '2021-08-27'"
+
+  def up
+    prepare_async_index TABLE, COLUMNS, name: INDEX_NAME, where: CONSTRAINTS
+  end
+
+  def down
+    unprepare_async_index TABLE, COLUMNS, name: INDEX_NAME, where: CONSTRAINTS
+  end
+end

db/schema_migrations/20211028123412

+d7655fb2a01e389f59990b395ed7ef683b21a3371b7e68d3eb29def85eea1888
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