Prometheus labels

Hello Team,
I’am using prometheus agent-mode to send metrics to the centralized one , and i want to filter metrics based on cluster,
i added this to agent-prom:

prometheus:
  prometheusSpec:
    externalLabels:
       cluster: agent-cluster

and this to central-prom:

prometheus:
  prometheusSpec:
     externalLabels:
         cluster: central-cluster

The problem is when i try: up{cluster=“”} in the central prom UI i can only see agent-cluster and central-cluster isn’t there !