Dedup issue_metrics table

What does this MR do?

This MR deduplicates the issue_metrics table in a similar fashion like !29566 (merged)

The MR does the following:

1. The application code ensures that issue_metrics records are created with the safe find_or_create method to avoid RecordNotUnique errors (if metrics records are created concurrently).
2. Add a new unique index with the highest issue_id which prevents further duplicates (very unlikely that we'll see duplicates).
3. Iterate over issue_metrics with EachBatch and find duplicated entries.
4. Eliminate the duplicated rows by "merging" them.
5. Add global unique index on issue_id and remove the tmp index created by step 2.

Migration

Up:

== 20210226141517 DedupIssueMetrics: migrating ================================
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:issue_metrics, :issue_id, {:where=>"id > 504", :unique=>true, :name=>"tmp_unique_issue_metrics_by_issue_id", :algorithm=>:concurrently})
   -> 0.0020s
-- execute("SET statement_timeout TO 0")
   -> 0.0004s
-- add_index(:issue_metrics, :issue_id, {:where=>"id > 504", :unique=>true, :name=>"tmp_unique_issue_metrics_by_issue_id", :algorithm=>:concurrently})
   -> 0.0182s
-- execute("RESET ALL")
   -> 0.0005s
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:issue_metrics, :issue_id, {:unique=>true, :name=>"index_unique_issue_metrics_issue_id", :algorithm=>:concurrently})
   -> 0.0018s
-- add_index(:issue_metrics, :issue_id, {:unique=>true, :name=>"index_unique_issue_metrics_issue_id", :algorithm=>:concurrently})
   -> 0.0035s
-- transaction_open?()
   -> 0.0000s
-- indexes(:issue_metrics)
   -> 0.0018s
-- remove_index(:issue_metrics, {:algorithm=>:concurrently, :name=>"tmp_unique_issue_metrics_by_issue_id"})
   -> 0.0015s
-- transaction_open?()
   -> 0.0000s
-- indexes(:issue_metrics)
   -> 0.0014s
-- remove_index(:issue_metrics, {:algorithm=>:concurrently, :name=>"index_issue_metrics"})
   -> 0.0018s
== 20210226141517 DedupIssueMetrics: migrated (0.0476s) =======================

Down:

== 20210226141517 DedupIssueMetrics: reverting ================================
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:issue_metrics, :issue_id, {:name=>"index_issue_metrics", :algorithm=>:concurrently})
   -> 0.0028s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- add_index(:issue_metrics, :issue_id, {:name=>"index_issue_metrics", :algorithm=>:concurrently})
   -> 0.0184s
-- execute("RESET ALL")
   -> 0.0016s
-- transaction_open?()
   -> 0.0000s
-- indexes(:issue_metrics)
   -> 0.0060s
-- current_schema()
   -> 0.0005s
-- transaction_open?()
   -> 0.0000s
-- indexes(:issue_metrics)
   -> 0.0062s
-- remove_index(:issue_metrics, {:algorithm=>:concurrently, :name=>"index_unique_issue_metrics_issue_id"})
   -> 0.0039s
== 20210226141517 DedupIssueMetrics: reverted (0.0498s) =======================

Database

Queries:

• Iteration query (using pkey and EachBatch): https://explain.depesz.com/s/waD8d
• Duplication finder query: https://explain.depesz.com/s/eJKL

Timing calculation:

(32_000_000 rows / BATCH_SIZE_1000) * (2 queries, ~10ms) = < 6 minutes

The calculation does not contain the deduplication time. IMHO it's negligible since we don't have too many duplicated rows (6 rows).

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #214456 (closed)