Prometheus starts with no exporters. What can you query already?

Why this is the answer

A default Prometheus scrapes its own /metrics endpoint, so prometheus_tsdb_*, prometheus_http_* and the rest are queryable immediately — which is how you check that the server is healthy before anything else exists. The TSDB is therefore not empty, host metrics need node_exporter as a separate process, and up is one of many series rather than the only one.