log info when endpoints change for a balancer

rootfs/etc/nginx/lua/balancer/resty.lua

@@ -1,5 +1,9 @@
 local util = require("util")
 
+local string_format = string.format
+local ngx_log = ngx.log
+local INFO = ngx.INFO
+
 local _M = {}
 
 function _M.new(self, o)
@@ -19,6 +23,8 @@ function _M.sync(self, backend)
     return
   end
 
+  ngx_log(INFO, string_format("[%s] nodes have changed for backend %s", self.name, backend.name))
+
   self.instance:reinit(nodes)
 end