Update series metadata

Ensure that EOL releases are removed from metadata
and tests, and that the current dev release is enabled
in metadata.  Dev release tests are enabled separately
because of chickens and eggs.

Change-Id: Idd527f75de70e43006ed6145b3599361e055e4c1

src/metadata.yaml

@@ -13,6 +13,7 @@ series:
   - xenial
   - bionic
   - cosmic
+  - disco
 provides:
   metric-service:
     interface: gnocchi