ctrl: f_counter_name_vals_get_n(): add start_idx, for hnbgw

Allow starting with a specific 'msc' / 'sgsn' instance without having to
read all the lower numbers along. For HNBGW_Tests.ttcn.

Change-Id: I9b74a1df9e115883b4b0ac0f606a370c6aca7f40
diff --git a/library/Osmocom_CTRL_Functions.ttcn b/library/Osmocom_CTRL_Functions.ttcn
index 2c90de4..75f8646 100644
--- a/library/Osmocom_CTRL_Functions.ttcn
+++ b/library/Osmocom_CTRL_Functions.ttcn
@@ -233,10 +233,11 @@
 
 	/* Retrieve the first N instances' rate counter values of the given names */
 	function f_counter_name_vals_get_n(IPA_CTRL_PT pt, charstring instance_name := "msc",
-					   integer instance_count, CounterNameVals counternames)
+					   integer instance_count, CounterNameVals counternames,
+					   integer start_idx := 0)
 	return CounterNameValsList {
 		var CounterNameValsList valslist;
-		for (var integer instance_nr := 0; instance_nr < instance_count; instance_nr := instance_nr + 1) {
+		for (var integer instance_nr := start_idx; instance_nr < start_idx + instance_count; instance_nr := instance_nr + 1) {
 			valslist[instance_nr] := f_counter_name_vals_get(pt, instance_name, instance_nr, counternames);
 		}
 		log("retrieved rate counters: ", instance_name, ": ", valslist);