add a list of potential exporters

howto/prometheus.md

@@ -642,6 +642,20 @@ Note that this list might become out of sync with the actual
 implementation, look into [Puppet](howto/puppet) in
 `profile::prometheus::server::external` for the actual deployment.
 
+### Other possible services to monitor
+
+Many more exporters could be configured. A non-exaustive list was
+built in [ticket tpo/tpa/team#30028](https://gitlab.torproject.org/tpo/tpa/team/-/issues/30028) around launch time. Here we
+can document more such exporters we find along the way:
+
+ * [Prometheus Onion Service Exporter](https://github.com/systemli/prometheus-onion-service-exporter/) - "Export the status and
+   latency of an onion service"
+ * [hsprober](https://git.autistici.org/ale/hsprober) - similar, but also with histogram buckets, multiple
+   attempts, warm-up and error counts
+ * [haproxy_exporter](https://github.com/prometheus/haproxy_exporter)
+
+There's also a [list of third-party exporters](https://prometheus.io/docs/instrumenting/exporters/) in the Prometheus documentation.
+
 ## SLA
 
 Prometheus is currently not doing alerting so it doesn't have any sort