Add JSON log example

docs/examples/json-log/README.md

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+# JSON Log Example
+
+RabbitMQ 3.9 switched from [Lager](https://github.com/erlang-lager/lager) to the new Erlang [Logger API](https://erlang.org/doc/man/logger.html) supporting JSON-formatted messages.
+
+Pull Requests:
+* [Switch from Lager to the new Erlang Logger API for logging #2861](https://github.com/rabbitmq/rabbitmq-server/pull/2861)
+* [Logging: Add configuration variables to set various formats #2927](https://github.com/rabbitmq/rabbitmq-server/pull/2927)
+
+This example configures RabbitMQ to output JSON logs.
+
+You can deploy this example like this:
+
+```shell
+kubectl apply -f rabbitmq.yaml
+```
+
+And once deployed, you can see the JSON logs:
+
+```shell
+kubectl logs json-server-0 -c rabbitmq
+```

docs/examples/json-log/rabbitmq.yaml

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+apiVersion: rabbitmq.com/v1beta1
+kind: RabbitmqCluster
+metadata:
+  name: json
+spec:
+  # JSON logging is supported from RabbitMQ 3.9 onwards
+  image: rabbitmq:3.9-management
+  rabbitmq:
+    # Disable RABBITMQ_LOGS=- which is set in
+    # https://github.com/docker-library/rabbitmq/blob/ece63d4534cc44abd6b1ec35bbd9aa0d21e87e1d/3.9/ubuntu/Dockerfile#L211
+    # Otherwise, this environment variable will override all log configurations in additionalConfig.
+    envConfig: |
+      RABBITMQ_LOGS=""
+    # More log configuration options can be found in https://github.com/rabbitmq/rabbitmq-server/pull/2927
+    additionalConfig: |
+      log.console = true
+      log.console.level = debug
+      log.console.formatter = json
+      log.console.formatter.json.field_map = level time msg domain file line pid
+      log.console.formatter.time_format = rfc3339_T

docs/examples/json-log/test.sh

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+#!/bin/bash
+
+set -eo pipefail
+
+kubectl logs json-server-0 -c rabbitmq --tail=3 | jq .