register_meta() calls should use 'object_subtype' argument

[r-a-y]

bbPress registers various meta keys, but doesn't set the 'object_subtype' argument:
​https://bbpress.trac.wordpress.org/browser/branches/2.6/src/bbpress.php?marks=778-787#L768

Currently, this means any post that is published will receive the following meta keys regardless of bbPress post type:

• _bbp_topic_count
• _bbp_reply_count
• _bbp_total_topic_count
• _bbp_total_reply_count
• _bbp_voice_count
• _bbp_anonymous_reply_count
• _bbp_topic_count_hidden
• _bbp_reply_count_hidden
• _bbp_forum_subforum_count

I've attached a patch that only registers meta keys for the correct bbPress forum and topic post types.

[r-a-y]

Could also use register_post_meta() in place of register_meta() instead.

Ignore 01.patch, use 02.patch.

[r-a-y]

#3588 was marked as a duplicate.

[r-a-y]

Bumping this one as this was mentioned recently in #3588.

[johnjamesjacoby]

Let's not use register_post_meta().

There is no register_user_meta() equivalent, and I'd like to keep the code patterned similarly if possible.

I'll be committing a modified version of @r-a-y's first patch, because a few of the meta keys are available to both forums and topics.

[johnjamesjacoby]

In 7351:

Meta: pass post-type into relevant calls to register_meta().

This change ensures that these bbPress meta keys are not associated with other custom post-types, particularly when using the REST API.

Props r-a-y, terresquall.

See #3588.

Fixes #3436.