Add initial query for sequence usage

sample-config/pgmon-metrics.yml

@@ -196,6 +196,27 @@ metrics:
     test_args:
       dbname: postgres
 
+  sequence_usage:
+    type: value
+    query:
+# 9.2 lacks lateral joins, the pg_sequence_last_value function, and the pg_sequences view
+#      0: >
+#        SELECT COALESCE(MAX(pg_sequence_last_value(c.oid)::float / (pg_sequence_parameters(oid)).maximum_value), 0) AS max_usage
+#        FROM pg_class c
+#        WHERE c.relkind = 'S'
+# 9.3 - 9.6 lacks the pg_sequence_last_value function, and pg_sequences view
+#      90300: >
+#        SELECT COALESCE(MAX(pg_sequence_last_value(c.oid)::float / s.maximum_value), 0) AS max_usage
+#        FROM pg_class c
+#        CROSS JOIN LATERAL pg_sequence_parameters(c.oid) AS s
+#        WHERE c.relkind = 'S'
+      100000: >
+        SELECT COALESCE(MAX(pg_sequence_last_value(c.oid)::float / sp.maximum_value), 0) AS max_usage
+        FROM pg_class c
+        CROSS JOIN LATERAL pg_sequence_parameters(c.oid) AS sp
+        WHERE c.relkind = 'S'
+    test_args:
+      dbname: postgres
 
   ##
   # Per-replication metrics