Comment 14 for bug 1820279

Tested with stein-proposed:

 apt policy apt policy libapache2-mod-auth-mellon

apt:
  Installed: 1.6.11
  Candidate: 1.6.11
  Version table:
 *** 1.6.11 500
        500 http://archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages
        100 /var/lib/dpkg/status

# grep MellonDiag /etc/apache2/mods-enabled/auth_mellon.conf
MellonDiagnosticsFile /var/log/apache2/mellon_diagnostics.log
MellonDiagnosticsEnable On

sudo tail -f /var/log/apache2/mellon_diagnostics.log
---------------------------------- New Request ---------------------------------
GET - /v3/OS-FEDERATION/identity_providers/samltestid/protocols/saml2/auth
log_id: (null)
server: scheme=(null) hostname=keystone.maas port=0
pid: 1668880, tid: 140134808790784
unparsed_uri: /v3/OS-FEDERATION/identity_providers/samltestid/protocols/saml2/auth
uri: /v3/OS-FEDERATION/identity_providers/samltestid/protocols/saml2/auth
path_info: (null)
filename: proxy:http://localhost:4980/v3/OS-FEDERATION/identity_providers/samltestid/protocols/saml2/auth
query args: (null)
Request Headers:
  Host: keystone.maas:5000
  User-Agent: openstacksdk/0.26.0 keystoneauth1/3.13.1 python-requests/2.18.4 CPython/3.6.7
  Accept-Encoding: gzip, deflate
  Accept: */*,application/vnd.paos+xml
  Connection: keep-alive
  PAOS: ver="urn:liberty:paos:2003-08";"urn:oasis:names:tc:SAML:2.0:profiles:SSO:ecp"
Mellon Directory Configuration for URL: /v3/OS-FEDERATION/identity_providers/samltestid/protocols/saml2/auth
# ...