charm metadata support for subordinates

docs/source/drafts/implicit-relations.rst

+ Implicit relations
+===================
+
+Implicit relations allow for interested services to gather
+lifecycle-oriented events and data about other services without
+expecting or requiring any modifications on the part of the author of
+the other service's charm.
+
+Implicit relationships are named in the reserved the juju-*
+namespace. Both the relation name and interface names provided by juju
+are prefixed with `juju-`. Charms attempting to provide new
+relationships in this namespace will trigger an error.
+
+Juju currently provides one implicit relationship to all deployed
+services.
+
+`juju-info`, if specified would look like::
+
+   provides:
+       juju-info:
+           interface: juju-info
+
+The charm author should not declare the `juju-info` relation and is
+provided here only as an example. The `juju-info` relation is
+implicitly provided by all charms, and enables the requiring unit to
+obtain basic details about the related-to unit. The following settings
+will be implicitly provided by the remote unit in a relation through its
+`juju-info` relation ::
+
+     private-address
+     public-address
+
+
+Relationship resolution
+-----------------------
+
+If **rsyslog** is a :doc:`subordinate charm<subordinate-services>` and
+requires a valid `scope: container` relationship in order to
+deploy. It can take advantage of optional support from the principal
+charm but in the event that the principal charm doesn't provide this
+support it will still require a `scope: container` relationship. In
+this event the logging charm author can take advantage of the implicit
+relationship offered by all charms, `juju-info`. ::
+
+   requires:
+     logging:
+         interface: logging-directory
+         scope: container
+     juju-info:
+         interface: juju-info
+         scope: container
+
+The admin then issues the following  ::
+
+    juju add-relation wordpress rsyslog
+
+If the wordpress author doesn't provide the `logging-directory`
+interface juju will use the less-specific (in the sense that it likely
+provides less information) `juju-info` interface.
+
+juju always attempts to match user provided interfaces before looking
+for possible relationship matches in the `juju-*` namespace.