[SAML] Allow to override metadata file name on Saml2MetadataFilter

[GitHanter]

Currently writeMetadataToResponse method on Saml2MetadataFilter is private, we may need it protected, so we can allow developer to customize the SP metadata file name.

[jzheaux]

Thanks for the suggestion, @GitHanter.

How would your application determine the name of the file? I'm wondering if something more involved than Saml2MetadataFilter#setMetadataFilename is needed.

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[GitHanter]

Thanks for the suggestion, @GitHanter.

How would your application determine the name of the file? I'm wondering if something more involved than Saml2MetadataFilter#setMetadataFilename is needed.

We name it as sp-<registrationId>-metadata.xml, but I think we should give developer a chance to change the metadata file name.