This extension exposes [Druid metrics](https://druid.apache.org/docs/latest/operations/metrics.html) for collection by a Prometheus server (https://prometheus.io/).
Emitter is enabled by setting `druid.emitter=prometheus` [configs](https://druid.apache.org/docs/latest/configuration/index.html#enabling-metrics) or include `prometheus` in the composing emitter list.
| `druid.emitter.prometheus.strategy` | The strategy to expose prometheus metrics. <br/>Should be one of `exporter` and `pushgateway`. Default strategy `exporter` would expose metrics for scraping purpose. Peon tasks (short-lived jobs) should use `pushgateway` strategy. | yes | exporter |
| `druid.emitter.prometheus.port` | The port on which to expose the prometheus HTTPServer. Required if using `exporter` strategy. | no | none |
| `druid.emitter.prometheus.namespace` | Optional metric namespace. Must match the regex `[a-zA-Z_:][a-zA-Z0-9_:]*` | no | druid |
| `druid.emitter.prometheus.dimensionMapPath` | JSON file defining the Prometheus metric type, desired dimensions, help text, and conversionFactor for every Druid metric. | no | Default mapping provided. See below. |
| `druid.emitter.prometheus.addHostAsLabel` | Flag to include the hostname as a prometheus label. | no | false |
| `druid.emitter.prometheus.addServiceAsLabel` | Flag to include the druid service name (e.g. `druid/broker`, `druid/coordinator`, etc.) as a prometheus label. | no | false |
| `druid.emitter.prometheus.pushGatewayAddress` | Pushgateway address. Required if using `pushgateway` strategy. | no | none |
In certain instances, Druid processes may be colocated on the same host. For example, the Broker and Router may share the same server. Other colocated processes include the Historical and MiddleManager or the Coordinator and Overlord. When you have colocated processes, specify `druid.emitter.prometheus.port` separately for each process on each host. For example, even if the Broker and Router share the same host, the Broker runtime properties and the Router runtime properties each need to list `druid.emitter.prometheus.port`, and the port value for both must be different.
Peon tasks are created dynamically by middle managers and have dynamic host and port addresses. Since the `exporter` strategy allows Prometheus to read only from a fixed address, it cannot be used for peon tasks.
So, these tasks need to be configured to use `pushgateway` strategy to push metrics from Druid to prometheus gateway.
If this emitter is configured to use `exporter` strategy globally, some of the above configurations need to be overridden in the middle manager so that spawned peon tasks can still use the `pushgateway` strategy.