External metrics cannot be viewed on Advanced Data Exporation

I added external metrics(Postgres_exporter's metrics) to PMM, and in Prometheus I can see the values of these metrics, but I see "No data " errors on Advanced Data Exploration for these metrics.
For example, I checked pg_settings_cpu_operator_cost metrics in prometheus graph (see screenshot), but in Advanced Data Exploration dashboard it return "No datapoint" error.
But when I changed from

 $metric{instance=~"$host"} 

to

 $metric{instance="10.10.11.50:9187"}

it started working for external metrics