Adds a unique index for storage limit exclusions

What does this MR do and why?

A Namespaces::Storage::LimitExclusion belongs to a Namespace, we should only ever have one LimitExclusion per Namespace.

This MR replaces the index on namespaces_storage_limit_exclusions table with a unique one on namespace_id and adds a matching validation on the model to support that.

The table is currently empty/unused.

Refs https://gitlab.com/gitlab-org/gitlab/-/issues/404751

Migration info

migrate up

main: == 20230405102815 AddUniqueNamespaceIndexToStorageLimitExclusions: migrating ==
main: -- transaction_open?()
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0899s
main: -- index_exists?("namespaces_storage_limit_exclusions", :namespace_id, {:unique=>true, :name=>"unique_idx_namespaces_storage_limit_exclusions_on_namespace_id", :algorithm=>:concurrently})
main:    -> 0.0017s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0002s
main: -- add_index("namespaces_storage_limit_exclusions", :namespace_id, {:unique=>true, :name=>"unique_idx_namespaces_storage_limit_exclusions_on_namespace_id", :algorithm=>:concurrently})
main:    -> 0.0025s
main: -- execute("RESET statement_timeout")
main:    -> 0.0003s
main: -- transaction_open?()
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0003s
main: -- indexes("namespaces_storage_limit_exclusions")
main:    -> 0.0014s
main: -- remove_index("namespaces_storage_limit_exclusions", {:algorithm=>:concurrently, :name=>"index_namespaces_storage_limit_exclusions_on_namespace_id"})
main:    -> 0.0017s
main: == 20230405102815 AddUniqueNamespaceIndexToStorageLimitExclusions: migrated (0.1181s)

migrate down

main: == 20230405102815 AddUniqueNamespaceIndexToStorageLimitExclusions: reverting ==
main: -- transaction_open?()
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.1991s
main: -- index_exists?("namespaces_storage_limit_exclusions", :namespace_id, {:unique=>false, :name=>"index_namespaces_storage_limit_exclusions_on_namespace_id", :algorithm=>:concurrently})
main:    -> 0.0053s
main: -- execute("SET statement_timeout TO 0")
main:    -> 0.0009s
main: -- add_index("namespaces_storage_limit_exclusions", :namespace_id, {:unique=>false, :name=>"index_namespaces_storage_limit_exclusions_on_namespace_id", :algorithm=>:concurrently})
main:    -> 0.0062s
main: -- execute("RESET statement_timeout")
main:    -> 0.0024s
main: -- transaction_open?()
main:    -> 0.0000s
main: -- view_exists?(:postgres_partitions)
main:    -> 0.0045s
main: -- indexes("namespaces_storage_limit_exclusions")
main:    -> 0.0049s
main: -- remove_index("namespaces_storage_limit_exclusions", {:algorithm=>:concurrently, :name=>"unique_idx_namespaces_storage_limit_exclusions_on_namespace_id"})
main:    -> 0.0060s
main: == 20230405102815 AddUniqueNamespaceIndexToStorageLimitExclusions: reverted (0.2861s)

How to set up and validate locally

1. Create an initial exclusion

   Namespaces::Storage::LimitExclusion.create(namespace: your_namespace_here, reason: "testing")

   this should be successful
2. Then try and create a second for the same namespace:

   Namespaces::Storage::LimitExclusion.create(namespace: your_namespace_here, reason: "testing")

   this should raise the error

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.