Add configuration option for native Prometheus exporter

Zookeeper supports Prometheus metrics as documented in
https://zookeeper.apache.org/doc/r3.8.0/zookeeperMonitor.html#Prometheus

This patch adds configuration options to enable it and change
the default port.

Change-Id: I7017bb4c2eea855464989bf10d7984e130cad4b3

defaults/main.yml

@@ -160,3 +160,6 @@ zookeeper_commands_whitelist:
   - ruok
   - isro
   - envi
+
+zookeeper_prometheus_enable: False
+zookeeper_prometheus_port: 7000

templates/zoo.cfg.j2

@@ -56,3 +56,8 @@ ssl.quorum.enabledProtocols={{ zookeeper_ssl_protocols | join(',') }}
 ssl.quorum.trustStore.type=PEM
 ssl.quorum.clientAuth={{ zookeeper_ssl_quorum_client_auth }}
 {% endif %}
+
+{% if zookeeper_prometheus_enable | bool %}
+metricsProvider.className=org.apache.zookeeper.metrics.prometheus.PrometheusMetricsProvider
+metricsProvider.httpPort={{ zookeeper_prometheus_port }}
+{% endif %}