Set a official default port for prometheus service.

main/src/main/java/com/ongres/pgio/main/Main.java

@@ -194,11 +194,11 @@ public class Main {
         "Run as a prometheus service (advanced option)")
       .availableIf("advanced");
     parser.acceptsAll(Lists.newArrayList("prometheus-bind"), 
-        "The bind address of prometheus service (advanced option)")
+        "The bind address of prometheus service, default is localhost (advanced option)")
       .availableIf("advanced")
       .withRequiredArg();
     parser.acceptsAll(Lists.newArrayList("prometheus-port"), 
-        "The port of prometheus service (advanced option)")
+        "The port of prometheus service, default is 9544 (advanced option)")
       .availableIf("advanced")
       .withRequiredArg();
     parser.acceptsAll(Lists.newArrayList("ppid"), 

main/src/main/java/com/ongres/pgio/main/config/Config.java

@@ -112,7 +112,7 @@ public class Config {
     private Optional<Integer> ppid = Optional.empty();
     private boolean prometheusService;
     private InetAddress prometheusBind;
-    private int prometheusPort = 8000;
+    private int prometheusPort = 9544;
     private boolean noPrintHeader;
     private boolean showSystem;
     private boolean showOther;