Thanks Francis for your response.

I tried the approach you suggested and it still doesn't seem to work.
This is what I am doing right now :-

limit_req_zone $default_client_id zone=sample_zone:50k rate=3r/m sync;
map $client_id $default_client_id {
ZZZZZ "";
$client_id $client_id;
}

When I try to hit the gateway with ZZZZ client_id, it still limits the requests according to 3rpm configuration. Am I doing this wrong?

(PS - I'm pretty new to nginx, sorry if I'm asking dumb questions:) )