Handle situations where directories don't exist

- The pgsql_tmp and archive_ready directories don't always exist.  Since
  PG <=9.4 doesn't have the "missing_ok" parameter, we have to work
  around the missing directories explicitly.

sample-config/pgmon-metrics.yml

@@ -116,6 +116,59 @@ metrics:
         FROM pg_stat_io
         GROUP BY backend_type
 
+  temp_files:
+    type: row
+    query:
+      120000: >
+        SELECT count(*) AS count, coalesce(sum(size), 0) AS size
+        FROM pg_ls_tmpdir()
+        WHERE name LIKE 'pgsql_tmp%%'
+      90500: >
+        SELECT count(*) AS count, coalesce(sum((pg_stat_file(name)).size), 0) AS size
+        FROM pg_ls_dir('base/pgsql_tmp/', true, false) AS name
+        WHERE name LIKE 'pgsql_tmp%%'
+      0: >
+        SELECT
+          CASE WHEN (
+            SELECT count(*)
+            FROM pg_ls_dir('base') AS 
+            base WHERE base = 'pgsql_tmp') = 0
+          THEN 0
+          ELSE (
+            SELECT count(*)
+            FROM pg_ls_dir('base/pgsql_tmp/') AS name
+            WHERE name LIKE 'pgsql_tmp%%')
+          END
+
+
+  ready_archive_files:
+    type: value
+    query:
+      120000: >
+        SELECT count(*) AS count
+        FROM pg_ls_archive_statusdir()
+        WHERE name LIKE '%%.ready'
+      100000: >
+        SELECT count(*) AS count
+        FROM pg_ls_dir('pg_wal/archive_status/', true, false) AS name
+        WHERE name LIKE '%%.ready'
+      90500: >
+        SELECT count(*) AS count
+        FROM pg_ls_dir('pg_xlog/archive_status/', true, false) AS name
+        WHERE name LIKE '%%.ready'
+      0: >
+        SELECT
+          CASE WHEN (
+            SELECT count(*)
+            FROM pg_ls_dir('pg_xlog/') AS f
+            WHERE f = 'archive_status') = 0
+          THEN 0
+          ELSE (
+            SELECT count(*) AS count
+            FROM pg_ls_dir('pg_xlog/archive_status/') AS name
+            WHERE name LIKE '%%.ready')
+          END
+
 
   ##
   # Per-database metrics