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.

Prometheus provides /federate API, which can be used for returning all the scraped metrics. For example, the following curl query returns all the metrics in Prometheus text exposition format (e.g. in the same format as /metrics output), which were scraped recently:

curl -G http://demo.robustperception.io:9090/federate -d 'match[]={__name__!=""}'

P.S. VictoriaMetrics supports additional start and end query args, which can be used for returning metrics scraped at a specific time range - see these docs.