osmo-stat-dummy: add rate counters and statsd tester

It's handy for testing rate counters as well as statsd metrics.

Related: OS#5671
Change-Id: Ibd8b17aa3ba9ceb527c6231310f01d736fb542a7
diff --git a/.gitignore b/.gitignore
index 936773e..e62d4da 100644
--- a/.gitignore
+++ b/.gitignore
@@ -104,3 +104,4 @@
 
 contrib/libosmocore.spec
 /utils/osmo-ns-dummy
+utils/osmo-stat-dummy/osmo-stat-dummy