doc: Add IPv6 metadata address

The metadata service is going to be accessible over IPv6 too when
the following Neutron feature merges (still in Victoria, I hope):

https://bugs.launchpad.net/neutron/+bug/1460177

However all end-user facing metadata documentation is in Nova,
so unless we want to refactor/move this, let me propose this doc
change here.

Change-Id: I7e67680090da003e01e106be47b7a807164fecb3
Partial-Bug: #1460177

Author: rubasov

doc/source/admin/metadata-service.rst

@@ -25,7 +25,14 @@ the :ref:`cells V2 guide <cells-v2-layout-metadata-api>`.
    added in Stein. For versions prior to this release, you should not use the
    standalone :program:`nova-api-metadata` application for multiple cells.
 
-Guests access the service at ``169.254.169.254``. The networking service,
+Guests access the service at ``169.254.169.254`` or at ``fe80::a9fe:a9fe``.
+
+.. versionchanged:: 22.0.0
+
+    Starting with the Victoria release the metadata service is accessible
+    over IPv6 at the link-local address ``fe80::a9fe:a9fe``.
+
+The networking service,
 neutron, is responsible for intercepting these requests and adding HTTP headers
 which uniquely identify the source of the request before forwarding it to the
 metadata API server. For the Open vSwitch and Linux Bridge backends provided

doc/source/user/metadata.rst

@@ -72,8 +72,24 @@ The metadata service
 
 The *metadata service* provides a way for instances to retrieve
 instance-specific data via a REST API. Instances access this service at
-``169.254.169.254`` and all types of metadata, be it user-, nova- or
-vendor-provided, can be accessed via this service.
+``169.254.169.254`` or at ``fe80::a9fe:a9fe``. All types of metadata,
+be it user-, nova- or vendor-provided, can be accessed via this service.
+
+.. versionchanged:: 22.0.0
+
+    Starting with the Victoria release the metadata service is accessible
+    over IPv6 at the link-local address ``fe80::a9fe:a9fe``.
+
+.. note::
+
+    As with all IPv6 link-local addresses, the metadata IPv6
+    address is not complete without a zone identifier (in a Linux
+    guest that is usually the interface name concatenated after
+    a percent sign). Please also note that in URLs you should
+    URL-encode the percent sign itself. For example, assuming
+    that the primary network interface in the guest is ``ens2``
+    substitute ``http://[fe80::a9fe:a9fe%25ens2]:80/...`` for
+    ``http://169.254.169.254/...``.
 
 Using the metadata service
 ~~~~~~~~~~~~~~~~~~~~~~~~~~