Add max transaction duration to logs

What does this MR do and why?

This MR adds db_xxx_txn_max_duration_s keys to the logs to further support our investigation for an appropriate txn duration threshold.

See gitlab-com/gl-infra/scalability#2860 (closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

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Before	After

How to set up and validate locally

On rails console:

1. Initialise the store

[1] pry(main)> Gitlab::SafeRequestStore.begin!
=> true
[2] pry(main)> ::Gitlab::Metrics::Subscribers::ActiveRecord.db_counter_payload

2. Verify that max transaction duration is recorded

[3] pry(main)> ::Gitlab::Metrics::Subscribers::ActiveRecord.db_counter_payload[:db_main_txn_max_duration_s]
=> 0.0
[4] pry(main)> ::Gitlab::Metrics::Subscribers::ActiveRecord.db_counter_payload[:db_main_txn_duration_s]
=> 0.0
[5] pry(main)> ApplicationRecord.transaction { User.first; sleep 2; User.first; User.first }
  TRANSACTION (0.1ms)  BEGIN /*application:console,db_config_name:main,console_hostname:SylvestersMBP2.localdomain,console_username:sylvesterchin,line:/lib/gitlab/database/schema_cache_with_renamed_table_legacy.rb:23:in `primary_keys'*/
  User Load (3.9ms)  SELECT "users".* FROM "users" ORDER BY "users"."id" ASC LIMIT 1 /*application:console,db_config_name:main,console_hostname:SylvestersMBP2.localdomain,console_username:sylvesterchin,line:(pry):5:in `block in __pry__'*/
  User Load (1.7ms)  SELECT "users".* FROM "users" ORDER BY "users"."id" ASC LIMIT 1 /*application:console,db_config_name:main,console_hostname:SylvestersMBP2.localdomain,console_username:sylvesterchin,line:(pry):5:in `block in __pry__'*/
  User Load (1.0ms)  SELECT "users".* FROM "users" ORDER BY "users"."id" ASC LIMIT 1 /*application:console,db_config_name:main,console_hostname:SylvestersMBP2.localdomain,console_username:sylvesterchin,line:(pry):5:in `block in __pry__'*/
  TRANSACTION (1.0ms)  COMMIT /*application:console,db_config_name:main,console_hostname:SylvestersMBP2.localdomain,console_username:sylvesterchin,line:/lib/gitlab/database.rb:392:in `commit'*/
=> #<User id:1 @root>
[6] pry(main)> ::Gitlab::Metrics::Subscribers::ActiveRecord.db_counter_payload[:db_main_txn_max_duration_s]
=> 2.115
[7] pry(main)> ::Gitlab::Metrics::Subscribers::ActiveRecord.db_counter_payload[:db_main_txn_duration_s]
=> 2.115
[8] pry(main)> ApplicationRecord.transaction { User.first; sleep 4; User.first; User.first }
  TRANSACTION (0.6ms)  BEGIN /*application:console,db_config_name:main,console_hostname:SylvestersMBP2.localdomain,console_username:sylvesterchin,line:(pry):8:in `block in __pry__'*/
  User Load (1.6ms)  SELECT "users".* FROM "users" ORDER BY "users"."id" ASC LIMIT 1 /*application:console,db_config_name:main,console_hostname:SylvestersMBP2.localdomain,console_username:sylvesterchin,line:(pry):8:in `block in __pry__'*/
  User Load (2.2ms)  SELECT "users".* FROM "users" ORDER BY "users"."id" ASC LIMIT 1 /*application:console,db_config_name:main,console_hostname:SylvestersMBP2.localdomain,console_username:sylvesterchin,line:(pry):8:in `block in __pry__'*/
  User Load (1.6ms)  SELECT "users".* FROM "users" ORDER BY "users"."id" ASC LIMIT 1 /*application:console,db_config_name:main,console_hostname:SylvestersMBP2.localdomain,console_username:sylvesterchin,line:(pry):8:in `block in __pry__'*/
  TRANSACTION (1.1ms)  COMMIT /*application:console,db_config_name:main,console_hostname:SylvestersMBP2.localdomain,console_username:sylvesterchin,line:/lib/gitlab/database.rb:392:in `commit'*/
=> #<User id:1 @root>
[9] pry(main)> ::Gitlab::Metrics::Subscribers::ActiveRecord.db_counter_payload[:db_main_txn_max_duration_s]
=> 4.027
[10] pry(main)> ::Gitlab::Metrics::Subscribers::ActiveRecord.db_counter_payload[:db_main_txn_duration_s]
=> 6.142