Add pg_stat_io query

* Add a query for pg_stat_io metrics grouped by backend type

* Switch some unsupported values from 0 to NULL to better reflect their
  nonexistence

sample-config/pgmon-metrics.yml

@@ -49,7 +49,7 @@ metrics:
     query:
       0: >
         SELECT max(age(datfrozenxid)) AS xid_age,
-               0 AS mxid_age
+               NULL AS mxid_age
         FROM pg_database
       90600: >
         SELECT max(age(datfrozenxid)) AS xid_age,
@@ -86,6 +86,28 @@ metrics:
         CROSS JOIN pg_stat_checkpointer cp
 
 
+  io_per_backend:
+    type: set
+    query:
+      160000: >
+        SELECT backend_type,
+               COALESCE(SUM(reads), 0) AS reads,
+               COALESCE(SUM(read_time), 0) AS read_time,
+               COALESCE(SUM(writes), 0) AS writes,
+               COALESCE(SUM(write_time), 0) AS write_time,
+               COALESCE(SUM(writebacks), 0) AS writebacks,
+               COALESCE(SUM(writeback_time), 0) AS writeback_time,
+               COALESCE(SUM(extends), 0) AS extends,
+               COALESCE(SUM(extend_time), 0) AS extend_time,
+               COALESCE(SUM(op_bytes), 0) AS op_bytes,
+               COALESCE(SUM(hits), 0) AS hits,
+               COALESCE(SUM(evictions), 0) AS evictions,
+               COALESCE(SUM(reuses), 0) AS reuses,
+               COALESCE(SUM(fsyncs), 0) AS fsyncs,
+               COALESCE(SUM(fsync_time), 0) AS fsync_time
+        FROM pg_stat_io
+        GROUP BY backend_type
+
   ##
   # Per-database metrics
   ##
@@ -107,16 +129,16 @@ metrics:
                temp_files,
                temp_bytes,
                deadlocks,
-               0 AS checksum_failures,
+               NULL AS checksum_failures,
                blk_read_time,
                blk_write_time,
-               0 AS session_time,
+               NULL AS session_time,
-               0 AS active_time,
+               NULL AS active_time,
-               0 AS idle_in_transaction_time,
+               NULL AS idle_in_transaction_time,
-               0 AS sessions,
+               NULL AS sessions,
-               0 AS sessions_abandoned,
+               NULL AS sessions_abandoned,
-               0 AS sessions_fatal,
+               NULL AS sessions_fatal,
-               0 AS sessions_killed,
+               NULL AS sessions_killed,
                extract('epoch' from stats_reset)::float AS stats_reset
         FROM pg_stat_database WHERE datname = %(dbname)s
       140000: >
@@ -134,8 +156,7 @@ metrics:
                temp_files,
                temp_bytes,
                deadlocks,
-               COALESCE(checksum_failures,
+               COALESCE(checksum_failures, 0) AS checksum_failures,
-               0) AS checksum_failures,
                blk_read_time,
                blk_write_time,
                session_time,