+/**
+ * Update metadata for the specified object. If no value already exists for the specified object
+ * ID and metadata key, the metadata will be added.
+ *
+ * @since 2.9.0
+ * @uses $wpdb WordPress database object for queries.
+ * @uses do_action() Calls 'update_{$meta_type}_meta' before updating metadata with meta_id of
+ * metadata entry to update, object ID, meta key, and meta value
+ * @uses do_action() Calls 'updated_{$meta_type}_meta' after updating metadata with meta_id of
+ * updated metadata entry, object ID, meta key, and meta value
+ *
+ * @param string $meta_type Type of object metadata is for (e.g., comment, post, or user)
+ * @param int $object_id ID of the object metadata is for
+ * @param string $meta_key Metadata key
+ * @param string $meta_value Metadata value
+ * @param string $prev_value Optional. If specified, only update existing metadata entries with
+ * the specified value. Otherwise, update all entries.
+ * @return bool True on successful update, false on failure.
+ */