Introduce 'ActiveSupport::Notifications::Fanout::Subscribers::MonotonicTimed' and 'ActiveSupport::Notifications::monotonic_subscribe'

Also, change the signature of ‘ActiveSupport::Notifications::Fanout#subscribe’ to accept optional ‘monotonic’ boolean argument. Then initialize either a ‘Timed’ or ‘MonotonicTimed’ subscriber based on the value of ‘monotonic’ parameter.

Introduce ‘ActiveSupport::Notifications::monotonic_subscribe’ method

Also, provision ‘ActiveSupport::Notifications::subscribed’ to optionally accept ‘monotonic’ boolean argument.

Update documentation for ActiveSupport::Notifications

Add tests

Update guides documentation under the 'Active Support Instrumentation' chapter

Incorporate feedback: use optional keyword argument to specify optional 'monotonic' option to 'subscribed' method

Fix a typo

Hooks added:
- `service_download_chunk.active_storage`
- `service_update_metadata.active_storage`
- `preview.active_storage`

We added ActiveSupport::Notifications instrumentation of the processing of each middleware in the stack,
See 04ae0b0b.

See https://github.com/rails/rails/blob/master/activerecord/lib/active_record/connection_adapters/abstract_adapter.rb#L625.

Since e3f832a7 `ActionMailer::Base.receive` is
deprecated.

* Extend documentation of `ActiveSupport::Notifications.subscribe`

Add mention that a block with only one argument passed to the method
will yield an event object.

Related to #33451

* Emphasize that `SubscribeEventObjects` is a test class by adding suffix `Test`

Add mention about it to the "Active Support Instrumentation" guide

Related to https://github.com/rails/rails/pull/33751#discussion_r214745153
Follow up #33751

http links will be redirected to the https version, but still better to
just directly link to the https version.

[ci skip] A regular expression was used to find a lot of missing Oxford
commas and add them.  The regular expression was as follows.

", ([a-zA-Z0-9.\`:'\"]+ ){1,6}(or|and) "

* Added `service_delete_prefixed.active_storage`.

* `s/ddh/dhh/`

Need to explain the key is only there in certain cases.

Follow up to #26584

Follow up to #24115.

`A UNIQUE` we pronounce URL as 'yu-ni-k’. We use this everywhere. So, be consistent with it.

This is removed by this commit
cf: https://github.com/rails/rails/pull/5261

the hook was added in 2a0d97bc

- Changed `IN` to `ON` in all note sentences in guides.

References #18148.