metadata: add support for initial metadata

Add a metadata.values property that is used to fill the metadata store
with initial values.

Fixes #3076

src/daemon/minimal.conf.in

@@ -152,7 +152,15 @@ context.objects = [
     #{ factory = spa-node-factory   args = { factory.name = api.vulkan.compute.source node.name = my-compute-source } }
 
     # Make a default metadata store
-    { factory = metadata            args = { metadata.name = default } }
+    { factory = metadata
+        args = {
+            metadata.name = default
+    #       metadata.values = [
+    #            { key = default.audio.sink   value = { name = somesink } }
+    #            { key = default.audio.source value = { name = somesource } }
+    #        ]
+        }
+    }
 
     # A default dummy driver. This handles nodes marked with the "node.always-driver"
     # property when no other driver is currently active. JACK clients need this.

src/daemon/pipewire.conf.in

@@ -263,6 +263,17 @@ context.objects = [
     #        audio.position         = "FL,FR"
     #    }
     #}
+
+    # Use the metadata factory to create metadata and some default values.
+    #{ factory = metadata
+    #    args = {
+    #        metadata.name = my-metadata
+    #        metadata.values = [
+    #            { key = default.audio.sink   value = { name = somesink } }
+    #            { key = default.audio.source value = { name = somesource } }
+    #        ]
+    #    }
+    #}
 ]
 
 context.exec = [