Record issue metrics without subtransaction

What does this MR do?

Use UPSERT to create or update issue metrics so we do not need a subtransaction and multiple queries

I tested this with:

Issue.first.metrics.delete
Issue.first.save

Before:

  ...
  TRANSACTION (0.2ms)  SAVEPOINT active_record_1 /*application:console,line:/data/cache/bundle-2.7.2/ruby/2.7.0/gems/activerecord-6.1.3.2/lib/active_record/connection_adapters/abstract/savepoints.rb:11:in `create_savepoint'*/
  Issue::Metrics Load (0.3ms)  SELECT "issue_metrics".* FROM "issue_metrics" WHERE "issue_metrics"."issue_id" = $1 LIMIT $2 /*application:console,line:/data/cache/bundle-2.7.2/ruby/2.7.0/gems/activerecord-6.1.3.2/lib/active_record/connection_adapters/postgresql/database_statements.rb:53:in `exec_query'*/  [["issue_id", 1], ["LIMIT", 1]]
  Issue::Metrics Create (1.0ms)  INSERT INTO "issue_metrics" ("issue_id", "created_at", "updated_at") VALUES ($1, $2, $3) RETURNING "id" /*application:console,line:/data/cache/bundle-2.7.2/ruby/2.7.0/gems/activerecord-6.1.3.2/lib/active_record/connection_adapters/postgresql/database_statements.rb:53:in `exec_query'*/  [["issue_id", 1], ["created_at", "2021-08-19 02:48:46.629584"], ["updated_at", "2021-08-19 02:48:46.629584"]]
  TRANSACTION (0.2ms)  RELEASE SAVEPOINT active_record_1 /*application:console,line:/data/cache/bundle-2.7.2/ruby/2.7.0/gems/activerecord-6.1.3.2/lib/active_record/connection_adapters/abstract/savepoints.rb:19:in `release_savepoint'*/
  ...
  Issue::Metrics Update (0.5ms)  UPDATE "issue_metrics" SET "first_associated_with_milestone_at" = $1, "updated_at" = $2 WHERE "issue_metrics"."id" = $3 /*application:console,line:/data/cache/bundle-2.7.2/ruby/2.7.0/gems/activerecord-6.1.3.2/lib/active_record/connection_adapters/postgresql/database_statements.rb:70:in `exec_delete'*/  [["first_associated_with_milestone_at", "2021-08-19 02:48:46.632308"], ["updated_at", "2021-08-19 02:48:46.673098"], ["id", 451]]
  ...

After:

   ...
   (1.3ms)  INSERT INTO issue_metrics (issue_id, first_associated_with_milestone_at, first_added_to_board_at, created_at, updated_at)
VALUES (1, NOW(), NULL, NOW(), NOW())
ON CONFLICT (issue_id)
DO UPDATE SET
  first_associated_with_milestone_at = LEAST(issue_metrics.first_associated_with_milestone_at, EXCLUDED.first_associated_with_milestone_at),
  first_added_to_board_at = LEAST(issue_metrics.first_added_to_board_at, EXCLUDED.first_added_to_board_at),
  updated_at = NOW()
RETURNING id
 /*application:console,line:/app/models/issue/metrics.rb:28:in `record!'*/
 ...

So this also reduces the number of SQL queries in the worst case because previously we safe_find_or_create and then update the record. Now it would be a single upsert all the time.

In the best and most common case where the record exists, the old code would do a single select query while the new code will do a no-op upsert.

Related to #338815 (closed)

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• 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