All the metrics for a particular instance IP:9100 can be obtained via the following PromQL query:

{instance="IP:9100"}

This query returns all the metrics, which have the given instance="IP:9100" label. See time series selector docs for more details. See also PromQL tutorial.

The {instance="IP:9100"} query can be sent to the following Prometheus querying APIs:

• /api/v1/query - this endpoint returns matching time series values at the given timestamp specified via time query arg. For example, curl 'http://prometheus:9090/api/v1/query?query={instance="IP:9100"}' . Make sure you properly encoded query arg with percent encoding. See this demo.
• /api/v1/series - this endpoint returns matching time series without any data, e.g. only metric names and labels are returned. For example, curl 'http://prometheus:9090/api/v1/series?match[]={instance="IP:9100"}'. Make sure you properly encoded match[] arg with percent encoding. See this demo
• /api/v1/query_range - this endpoint returns calculated datapoints for matching time series on the selected time range [start ... end] with the given step interval between samples. See more details about calculated datapoints in these docs.

If you want returning just unique metric names without labels, then you can query /api/v1/label/__name__/values endpoint: curl http://prometheus:9090/api/v1/label/__name__/values . The __name__ is a special label name used in Prometheus for referring to metric names. Note that multiple time series may share the same metric name. For example, http_requests_total{path="/foo"} and http_requests_total{path="/bar"} time series share http_requests_total metric name, while they differ by path label values.