time filter for shdict get_keys

I have stumbled on a use case where I need to get to most recent items from the shared dict on the get_keys method. For example the ones added in the last 60 seconds. I didn't find a way yet, probably has to be implemented. I think of two ways.

1. Add a creation_time to the ngx_http_lua_shdict_node_t struct, but I didn't go through all the code to say that this is really needed. This doesn't seem favorable as it increases the memory usage,
2. add a parameter to get_keys function like ttl_remaining_above? so that this value can be used as a condition while getting the keys (expiry_time - now > param). This seems easier to implement and less taxing.

I think this would be a nice addition, what do you think?

Start by tracing the get_keys implementation and the ngx_http_lua_shdict_node_t struct to understand how shared-dict expiration is represented. Compare the proposed creation_time and remaining-TTL approaches, including their memory and API implications. Done means the desired recent-key behavior and its implementation scope are clearly decided.