Pass args from Zabbix explicitly

* Using $0 psases the entire command, so just pass all 9 possible
  positioinal values.

* Allow empty arguments.

* Update sample config:
** Use a socket by default.
** Return a set with database discovery to be able to assign a macro.

pgmon.cfg

@@ -1,5 +1,6 @@
-host=localhost
-port=54315
+host=/var/run/postgresql
+port=5432
+user=postgres
 metric=max_frozen_age:value::SELECT max(age(datfrozenxid)) FROM pg_database
 metric=db_stats:row::SELECT * FROM pg_stat_database WHERE datname = '{datname}'
-metric=discover_dbs:column::SELECT datname FROM pg_database
+metric=discover_dbs:set::SELECT datname FROM pg_database

pgmon.py

@@ -389,6 +389,7 @@ class Server:
 
                 if len(parts) > 1:
                     for arg in parts[1].split(','):
+                        if arg != '':
                             (k, v) = arg.split('=', 1)
                             args_dict[k] = v
             except socket.timeout:
@@ -500,7 +501,7 @@ class Worker(threading.Thread):
                 req.set_result(json.dumps(res[0]))
             elif metric.type == 'column':
                 req.set_result(json.dumps([list(r.values())[0] for r in res]))
-            else:
+            elif metric_type == 'set':
                 req.set_result(json.dumps(res))
 
             try:

pgmon_userparameter.conf

@@ -1 +1 @@
-UserParameter=pgmon[*],/usr/local/bin/pgmon.py -c /etc/zabbix/pgmon.cfg "$0"
+UserParameter=pgmon[*],/usr/local/bin/pgmon.py -c /etc/zabbix/pgmon.cfg "$1,$2,$3,$4,$5,$6,$7,$8,$9"