Switch from partition to routing table

What does this MR do and why?

It adds a way of switching the reads and writes through the routing table without reloading the models:

    partitionable scope: :build, through: {
      table: :p_ci_builds_metadata,
      flag: :ci_partitioning_use_ci_builds_metadata_routing_table
    }

Example queries for reading:

[1] pry(main)> Feature.disable(:ci_partitioning_use_ci_builds_metadata_routing_table)
=> true
[3] pry(main)> Ci::BuildMetadata.last; nil
  Ci::BuildMetadata Load (0.4ms)  SELECT "ci_builds_metadata".* FROM "ci_builds_metadata" ORDER BY "ci_builds_metadata"."id" DESC LIMIT 1 /*application:console,db_config_name:ci,line:(pry):3:in `__pry__'*/
=> nil
[4] pry(main)> Ci::Build.last.metadata; nil
  Ci::BuildMetadata Load (0.3ms)  SELECT "ci_builds_metadata".* FROM "ci_builds_metadata" WHERE "ci_builds_metadata"."build_id" = 6 LIMIT 1 /*application:console,db_config_name:ci,line:(pry):4:in `__pry__'*/
=> nil
[5] pry(main)> Feature.enable(:ci_partitioning_use_ci_builds_metadata_routing_table)
=> true
[6] pry(main)> Ci::BuildMetadata.last; nil
  Ci::BuildMetadata Load (0.5ms)  SELECT "p_ci_builds_metadata".* FROM "p_ci_builds_metadata" ORDER BY "p_ci_builds_metadata"."id" DESC LIMIT 1 /*application:console,db_config_name:ci,line:(pry):6:in `__pry__'*/
=> nil
[7] pry(main)> Ci::Build.last.metadata; nil
  Ci::BuildMetadata Load (0.2ms)  SELECT "p_ci_builds_metadata".* FROM "p_ci_builds_metadata" WHERE "p_ci_builds_metadata"."build_id" = 6 LIMIT 1 /*application:console,db_config_name:ci,line:(pry):7:in `__pry__'*/
=> nil
[8] pry(main)> Feature.disable(:ci_partitioning_use_ci_builds_metadata_routing_table)
=> true
[9] pry(main)> Ci::BuildMetadata.last; nil
  Ci::BuildMetadata Load (1.5ms)  SELECT "ci_builds_metadata".* FROM "ci_builds_metadata" ORDER BY "ci_builds_metadata"."id" DESC LIMIT 1 /*application:console,db_config_name:ci,line:(pry):9:in `__pry__'*/
=> nil
[10] pry(main)> Ci::Build.last.metadata; nil
  Ci::BuildMetadata Load (0.3ms)  SELECT "ci_builds_metadata".* FROM "ci_builds_metadata" WHERE "ci_builds_metadata"."build_id" = 6 LIMIT 1 /*application:console,db_config_name:ci,line:(pry):10:in `__pry__'*/
=> nil

Associations:

[7] pry(main)> Feature.enable(:ci_partitioning_use_ci_builds_metadata_routing_table)
=> true
[8] pry(main)> Ci::Build.last.metadata; nil
  Ci::Build Load (1.6ms)  SELECT "ci_builds"."status", "ci_builds"."finished_at", "ci_builds"."created_at", "ci_builds"."updated_at", "ci_builds"."started_at", "ci_builds"."runner_id", "ci_builds"."coverage", "ci_builds"."commit_id", "ci_builds"."name", "ci_builds"."options", "ci_builds"."allow_failure", "ci_builds"."stage", "ci_builds"."trigger_request_id", "ci_builds"."stage_idx", "ci_builds"."tag", "ci_builds"."ref", "ci_builds"."user_id", "ci_builds"."type", "ci_builds"."target_url", "ci_builds"."description", "ci_builds"."project_id", "ci_builds"."erased_by_id", "ci_builds"."erased_at", "ci_builds"."artifacts_expire_at", "ci_builds"."environment", "ci_builds"."when", "ci_builds"."yaml_variables", "ci_builds"."queued_at", "ci_builds"."lock_version", "ci_builds"."coverage_regex", "ci_builds"."auto_canceled_by_id", "ci_builds"."retried", "ci_builds"."protected", "ci_builds"."failure_reason", "ci_builds"."scheduled_at", "ci_builds"."token_encrypted", "ci_builds"."upstream_pipeline_id", "ci_builds"."resource_group_id", "ci_builds"."waiting_for_resource_at", "ci_builds"."processed", "ci_builds"."scheduling_type", "ci_builds"."id", "ci_builds"."stage_id", "ci_builds"."partition_id" FROM "ci_builds" WHERE "ci_builds"."type" = 'Ci::Build' ORDER BY "ci_builds"."id" DESC LIMIT 1 /*application:console,db_config_name:ci,line:(pry):2:in `__pry__'*/
  Ci::BuildMetadata Load (1.1ms)  SELECT "p_ci_builds_metadata".* FROM "p_ci_builds_metadata" WHERE "p_ci_builds_metadata"."build_id" = 35 LIMIT 1 /*application:console,db_config_name:ci,line:(pry):2:in `__pry__'*/
=> nil
[9] pry(main)> Ci::Build.preload(:metadata).last; nil
  Ci::Build Load (0.6ms)  SELECT "ci_builds"."status", "ci_builds"."finished_at", "ci_builds"."created_at", "ci_builds"."updated_at", "ci_builds"."started_at", "ci_builds"."runner_id", "ci_builds"."coverage", "ci_builds"."commit_id", "ci_builds"."name", "ci_builds"."options", "ci_builds"."allow_failure", "ci_builds"."stage", "ci_builds"."trigger_request_id", "ci_builds"."stage_idx", "ci_builds"."tag", "ci_builds"."ref", "ci_builds"."user_id", "ci_builds"."type", "ci_builds"."target_url", "ci_builds"."description", "ci_builds"."project_id", "ci_builds"."erased_by_id", "ci_builds"."erased_at", "ci_builds"."artifacts_expire_at", "ci_builds"."environment", "ci_builds"."when", "ci_builds"."yaml_variables", "ci_builds"."queued_at", "ci_builds"."lock_version", "ci_builds"."coverage_regex", "ci_builds"."auto_canceled_by_id", "ci_builds"."retried", "ci_builds"."protected", "ci_builds"."failure_reason", "ci_builds"."scheduled_at", "ci_builds"."token_encrypted", "ci_builds"."upstream_pipeline_id", "ci_builds"."resource_group_id", "ci_builds"."waiting_for_resource_at", "ci_builds"."processed", "ci_builds"."scheduling_type", "ci_builds"."id", "ci_builds"."stage_id", "ci_builds"."partition_id" FROM "ci_builds" WHERE "ci_builds"."type" = 'Ci::Build' ORDER BY "ci_builds"."id" DESC LIMIT 1 /*application:console,db_config_name:ci,line:(pry):3:in `__pry__'*/
  Ci::BuildMetadata Load (0.1ms)  SELECT "p_ci_builds_metadata".* FROM "p_ci_builds_metadata" WHERE "p_ci_builds_metadata"."build_id" = 35 /*application:console,db_config_name:ci,line:(pry):3:in `__pry__'*/
=> nil

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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.

Related to #377534 (closed)