Broken for me too, fly_volume_used_pct just never finds anything:
$ curl https://api.fly.io/prometheus/$ORG/api/v1/query_range?step=30 -H "Authorization: Bearer $FLY_AUTH_SECRET" --data-urlencode 'query=max (fly_volume_used_pct)'; echo
{"status":"success","isPartial":false,"data":{"resultType":"matrix","result":[]}}
I can query Fly’s other metrics just fine:
$ FLY_AUTH_SECRET="$(flyctl auth token)"
$ ORG=example_changeme
$ curl https://api.fly.io/prometheus/$ORG/api/v1/query_range?step=30 -H "Authorization: Bearer $FLY_AUTH_SECRET" --data-urlencode 'query=max (pg_stat_activity_count)'; echo
{"status":"success","isPartial":false,"data":{"resultType":"matrix","result":[{"metric":{},"values":[[1659993512,"17"],[1659993542,"17"],[1659993572,"17"],[1659993602,"9"],[1659993632,"11"],[1659993662,"11"],[1659993692,"11"],[1659993722,"11"],[1659993752,"11"],[1659993782,"12"],[1659993812,"12"]]}]}}