Fix activity query

* Modify the activity query to ensure each state is always reported.

* Fix the activity preprocessor to unwrap the value from the array
  containing it.

sample-config/pgmon-metrics.yml

@@ -196,12 +196,28 @@ metrics:
     type: set
     query:
       0: >
-        SELECT state,
-               count(*) AS backend_count,
-               COALESCE(EXTRACT(EPOCH FROM max(now() - state_change)), 0) AS max_state_time
-        FROM pg_stat_activity
-        WHERE datname = %(dbname)s
-        GROUP BY state
+        SELECT
+          states.state,
+          COALESCE(a.backend_count, 0) AS backend_count,
+          COALESCE(a.max_state_time, 0) AS max_state_time
+        FROM (
+          SELECT state,
+                 count(*) AS backend_count,
+                 COALESCE(EXTRACT(EPOCH FROM now() - min(state_change)), 0) AS max_state_time
+          FROM pg_stat_activity
+          WHERE datname = %(dbname)s
+          GROUP BY state
+        ) AS a
+        RIGHT JOIN
+          unnest(ARRAY[
+            'active',
+            'idle',
+            'idle in transaction',
+            'idle in transaction (aborted)',
+            'fastpath function call',
+            'disabled'
+          ]) AS states(state)
+        USING (state);
     test_args:
       dbname: postgres
 
@@ -235,7 +251,7 @@ metrics:
   locks:
     type: row
     query:
-      0:
+      0: >
         SELECT COUNT(*) AS total,
                SUM(CASE WHEN granted THEN 1 ELSE 0 END) AS granted
         FROM pg_locks