"""Constructs the metadata URL for a given resource URL following RFC 9728.

This function inserts '/.well-known/oauth-protected-resource' between the host and the path of the resource URL. If the resource URL contains a path, it is appended after the well-known suffix.

Args:
    resource_url (str): The URL of the resource for which the metadata URL is to be constructed.

Returns:
    str: The constructed metadata URL following the RFC 9728 specification.
"""