As described in the ReloadableServices documentation, the configuration is actually loaded from a bean whose name is specified by the property idp.service.metadata.resources, with the default value shibboleth.MetadataResolverResources (in turn defined in conf/services.xml to be a list with one entry: the file metadata-providers.xml)
You can, if you choose, override this with additional or different files or more advanced sources. Each resource usually is expected to supply a "top level"
It is also now possible to declare "by reference" metadata filters that are attached by reference, so it is possible to supply a resource whose top-level element is such a <MetadataFilter> in such cases.
If a specific relying party (as identified by a specific entityID) is duplicated in the metadata sources provided, then which precise entry is chosen is governed by the following rules: