fix(template) fix iterator in stream shdict (#7078)
fix(template) fix iterator in stream shdict (#7078)

`kong.pdk.node` iterrates through ngx.shared magic table to
get list of shdict metrics, implement the __pairs metamethod so
it's happy.

Co-authored-by: Datong Sun <88ea7fe5b18d86dd626610096bab9e86520f1a58@konghq.com>
Kong/kong,Kong/kong,Kong/kong