Document Zookeeper's ZOO_CFG_EXTRA

zookeeper/content.md

@@ -108,7 +108,17 @@ Defaults to `srvr`. Zookeeper's [`4lw.commands.whitelist`](https://zookeeper.apa
 
 ## Advanced configuration
 
-Not every Zookeeper configuration setting is exposed via the environment variables listed above. These variables are only meant to cover minimum configuration keywords and some often changing options. If [mounting your custom config file](#configuration) as a volume doesn't work for you, consider using `JVMFLAGS` environment variable. Many of the Zookeeper advanced configuration options can be set there using Java system properties in the form of `-Dproperty=value`. For example, you can use Netty instead of NIO (default option) as a server communication framework:
+### `ZOO_CFG_EXTRA`
+
+Not every Zookeeper configuration setting is exposed via the environment variables listed above. These variables are only meant to cover minimum configuration keywords and some often changing options. If [mounting your custom config file](#configuration) as a volume doesn't work for you, consider using `ZOO_CFG_EXTRA` environment variable. You can add arbitrary configuration parameters to Zookeeper configuration file using this variable. The following example shows how to enable Prometheus metrics exporter on port `7070`:
+
+```console
+$ docker run --name some-zookeeper --restart always -e ZOO_CFG_EXTRA="metricsProvider.className=org.apache.zookeeper.metrics.prometheus.PrometheusMetricsProvider metricsProvider.httpPort=7070" %%IMAGE%%
+```
+
+### `JVMFLAGS`
+
+Many of the Zookeeper advanced configuration options can be set there using Java system properties in the form of `-Dproperty=value`. For example, you can use Netty instead of NIO (default option) as a server communication framework:
 
 ```console
 $ docker run --name some-zookeeper --restart always -e JVMFLAGS="-Dzookeeper.serverCnxnFactory=org.apache.zookeeper.server.NettyServerCnxnFactory" %%IMAGE%%