add_{$meta_type}_meta
Action Hook: Fires immediately before meta of a specific type is added.
Source: wp-includes/meta.php:117
added_{$meta_type}_meta
Action Hook: Fires immediately after meta of a specific type is added.
Source: wp-includes/meta.php:156
update_metadata()
Function: Updates metadata for the specified object. If no value already exists for the specified object ID and metadata key, the metadata will be added.
Source: wp-includes/meta.php:182