Fix storing first_mentioned_in_commit_at attribute

What does this MR do and why?

This change fixes the way we calculate and store the first_mentioned_in_commit_at attribute. Instead of looking at the most recent commit date, the system will check the earlies commit date looking back to a maximum of 100 commits. The change also includes a BG migration that attempts to fix the existing records.

BG migration:

1. Batch over issue_metrics via issue_id where first_mentioned_in_commit_at was set later than 2019
2. For each batch, do sub-batching. For each issue_metrics_record:
3. Find merge_requests_closing_isues records related to the issue.
4. Look up the MR diffs, look at the most recent diff.
5. Look at the diff commits, take the authored_date column.
6. If the found authored_date is earlier than first_mentioned_in_commit_at, then update the issue_metrics row.

Database

Queries

• Scheduling query 1: https://explain.depesz.com/s/b8S9
• Scheduling query 2: https://explain.depesz.com/s/d6DX
• Update query: https://explain.depesz.com/s/U29q (timing jumps between 300ms and 1s)

Migration

The index creation took 2 minutes on db-lab.

Up:

== 20211004110500 AddTemporaryIndexToIssueMetrics: migrating ==================
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:issue_metrics, :issue_id, {:where=>"EXTRACT(YEAR FROM first_mentioned_in_commit_at) > 2019", :name=>"index_issue_metrics_first_mentioned_in_commit", :algorithm=>:concurrently})
   -> 0.0030s
-- execute("SET statement_timeout TO 0")
   -> 0.0006s
-- add_index(:issue_metrics, :issue_id, {:where=>"EXTRACT(YEAR FROM first_mentioned_in_commit_at) > 2019", :name=>"index_issue_metrics_first_mentioned_in_commit", :algorithm=>:concurrently})
   -> 0.0070s
-- execute("RESET statement_timeout")
   -> 0.0006s
== 20211004110500 AddTemporaryIndexToIssueMetrics: migrated (0.0161s) =========

== 20211004110927 ScheduleFixFirstMentionedInCommitAtJob: migrating ===========
-- Scheduled 1 FixFirstMentionedInCommitAt jobs with a maximum of 10000 records per batch and an interval of 120 seconds.

The migration is expected to take at least 120 seconds. Expect all jobs to have completed after 2021-10-04 17:42:20 UTC."
== 20211004110927 ScheduleFixFirstMentionedInCommitAtJob: migrated (0.0147s) ==

Down:

== 20211004110927 ScheduleFixFirstMentionedInCommitAtJob: reverting ===========
== 20211004110927 ScheduleFixFirstMentionedInCommitAtJob: reverted (0.0000s) ==


== 20211004110500 AddTemporaryIndexToIssueMetrics: reverting ==================
-- transaction_open?()
   -> 0.0000s
-- indexes(:issue_metrics)
   -> 0.0025s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- remove_index(:issue_metrics, {:algorithm=>:concurrently, :name=>"index_issue_metrics_first_mentioned_in_commit"})
   -> 0.0059s
-- execute("RESET statement_timeout")
   -> 0.0006s
== 20211004110500 AddTemporaryIndexToIssueMetrics: reverted (0.0134s) =========

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 #339508 (closed)