nixos/logstash: add option examples

nixos/modules/services/logging/logstash.nix

@@ -20,16 +20,39 @@ in
       inputConfig = mkOption {
         default = ''stdin { type => "example" }'';
         description = "Logstash input configuration";
+        example = ''
+          # Read from journal
+          pipe {
+            command => "${pkgs.systemd}/bin/journalctl -f -o json"
+            type => "syslog" codec => json {}
+          }
+        '';
       };
 
       filterConfig = mkOption {
         default = ''noop {}'';
         description = "logstash filter configuration";
+        example = ''
+          if [type] == "syslog" {
+            # Keep only relevant systemd fields
+            # http://www.freedesktop.org/software/systemd/man/systemd.journal-fields.html
+            prune {
+              whitelist_names => [
+                "type", "@timestamp", "@version",
+                "MESSAGE", "PRIORITY", "SYSLOG_FACILITY",
+              ]
+            }
+          }
+        '';
       };
 
       outputConfig = mkOption {
         default = ''stdout { debug => true debug_format => "json"}'';
         description = "Logstash output configuration";
+        example = ''
+          redis { host => "localhost" data_type => "list" key => "logstash" codec => json }
+          elasticsearch { embedded => true }
+        '';
       };
     };
   };