The monitor is the Hub's application monitoring service. It collects and stores metrics on the health of the other services in the Hub (and on its self). Time series of these metrics can be queried and displayed.
We use Prometheus, a popular open-source application monitoring project orignally developed at SoundCloud.
Other services use so called "exporters" to expose metrics that Prometheus periodically scrapes, usually from the /metrics endpoint. For example, the manager uses the django_prometheus Python package as an exporter and the broker uses the rabbitmq_prometheus plugin.
Prometheus exposes a simple tool for visualization of metrics at http://localhost:9090/graph. For example, a graph of the up metric for each of the services monitored:
http://localhost:9090/internal/monitor/graph?g0.range_input=1h&g0.expr=up&g0.tab=0
Prometheus also exposes a HTTP API which can be queried to obtain time series of current values of metrics e.g.
http://localhost:9090/internal/monitor/api/v1/query?query=up
The router provides a reverse proxy to the monitor's graph interface and API at /internal/monitor.
Prometheus has a detailed comparison with alternatives.