Delete merge_request_metrics -> ci_pipelines FK

What does this MR do and why?

This MR removes the foreign key between the merge_request_metrics and ci_pipelines tables. This is needed for CI DB decomposition.

Within this MR, I add a new shared example that can test the loose FK cleanup behaviour.

Related epic: #348272 (closed)

Migration

Up:

== 20211213102111 DropCiPipelinesMrMetricsFk: migrating =======================
-- transaction_open?()
   -> 0.0000s
-- foreign_keys(:merge_request_metrics)
   -> 0.0045s
-- remove_foreign_key(:merge_request_metrics, :ci_pipelines, {:name=>"fk_rails_33ae169d48"})
   -> 0.0068s
== 20211213102111 DropCiPipelinesMrMetricsFk: migrated (0.0227s) ==============

Down:

== 20211213102111 DropCiPipelinesMrMetricsFk: reverting =======================
-- transaction_open?()
   -> 0.0000s
-- foreign_keys(:merge_request_metrics)
   -> 0.0026s
-- transaction_open?()
   -> 0.0000s
-- execute("ALTER TABLE merge_request_metrics\nADD CONSTRAINT fk_rails_33ae169d48\nFOREIGN KEY (pipeline_id)\nREFERENCES ci_pipelines (id)\nON DELETE CASCADE\nNOT VALID;\n")
   -> 0.0026s
-- execute("SET statement_timeout TO 0")
   -> 0.0007s
-- execute("ALTER TABLE merge_request_metrics VALIDATE CONSTRAINT fk_rails_33ae169d48;")
   -> 0.0057s
-- execute("RESET statement_timeout")
   -> 0.0006s
== 20211213102111 DropCiPipelinesMrMetricsFk: reverted (0.0254s) ==============

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