postgres: Do not log timestamp

By default, postgres prefixes each log line with a timestamp. On NixOS
logs are written to journal anyway, so they include an external
timestamp, so the timestamp ends up being printed twice, which clutters
the log.

* Add a module option to change the log prefix.
* Set it to upstream default sans timestamp.

nixos/modules/services/databases/postgresql.nix

@@ -17,6 +17,7 @@ let
       hba_file = '${pkgs.writeText "pg_hba.conf" cfg.authentication}'
       ident_file = '${pkgs.writeText "pg_ident.conf" cfg.identMap}'
       log_destination = 'stderr'
+      log_line_prefix = '${cfg.logLinePrefix}'
       listen_addresses = '${if cfg.enableTCPIP then "*" else "localhost"}'
       port = ${toString cfg.port}
       ${cfg.extraConfig}
@@ -192,6 +193,17 @@ in
         '';
       };
 
+      logLinePrefix = mkOption {
+        type = types.str;
+        default = "[%p] ";
+        example = "%m [%p] ";
+        description = ''
+          A printf-style string that is output at the beginning of each log line.
+          Upstream default is '%m [%p] ', i.e. it includes the timestamp. We do
+          not include the timestamp, because journal has it anyway.
+        '';
+      };
+
       extraPlugins = mkOption {
         type = types.listOf types.path;
         default = [];