exometer_report: Fix static subscriber spec with Extra param

As per documentation in [1], it is possible to pass an Extra param:
"{select, {MatchPattern, DataPoint, Interval [, Retry [, Extra] ]}}"
Example:
"""
{select, {[{ {['_' | '_'],'_','_'}, [], ['$_']}],
          exometer_report_statsd,
          value,
          1000,
	  true,
          [{report_type, counter}]
	 }
}
"""

Without this patch, exometer_report.erl was failing with log warning
"Incorrect static subscriber spec".

[1] https://github.com/Feuerlabs/exometer_core/tree/master#configuring-static-subscriptions

src/exometer_report.erl

@@ -1731,9 +1731,9 @@ init_subscriber({apply, {M, F, A}}) ->
     lists:foreach(fun(Sub) ->
                           init_subscriber(Sub)
                   end, apply(M, F, A));
-init_subscriber({select, Expr}) when tuple_size(Expr)==3;
-                                     tuple_size(Expr)==4;
-                                     tuple_size(Expr)==5 ->
+init_subscriber({select, Expr}) when tuple_size(Expr)==4;
+                                     tuple_size(Expr)==5;
+                                     tuple_size(Expr)==6 ->
     {Pattern, Reporter, DataPoint, Interval, Retry, Extra} =
         case Expr of
             {P, R, D, I} -> {P, R, D, I, true, undefined};
@@ -1750,7 +1750,7 @@ init_subscriber({select, Expr}) when tuple_size(Expr)==3;
 
 init_subscriber(Other) ->
     ?log(warning, "Incorrect static subscriber spec ~p. "
-             "Use { Reporter, Metric, DataPoint, Interval [, Extra ]}~n",
+             "Use { Reporter, Metric, DataPoint [, Interval [, Extra ] ]}~n",
              [ Other ]).
 
 get_reporter_status(R) ->