Add Kafka README for manual testing (#15955) (#16053)

metricbeat/module/kafka/README.md

@@ -0,0 +1,101 @@
+### Manually testing Kafka modules
+
+Testing Kafka can be tricky, so the purpose of this doc is to document all the steps that one should follow in order to
+prepare an environment and manually test Kafka module.
+
+#### Kafka container
+
+In order to have a Kafka instance up and running the best way to go is to use the container that is used by the CI tests.
+To bring this container up simply run the tests for Kafka module:
+
+`go test -tags integration ./metricbeat/module/kafka/...`
+
+
+After the tests have been completed, the Kafka container should be still running. Verify with:
+
+```console
+707b50334835    docker.elastic.co/integrations-ci/beats-kafka:2.1.1-2  "/run.sh"        2 minutes ago    Up 2 minutes (healthy)  2181/tcp, 0.0.0.0:32785->8774/tcp, 0.0.0.0:32784->8775/tcp, 0.0.0.0:32783->8779/tcp, 0.0.0.0:32782->9092/tcp  kafka_a035cf4c6889705a_kafka_1
+```
+
+In order to identify to which port the Broker is listening on one should check in the logs of the container and find 
+the advertised address:
+
+```console
+docker logs 707b50334835 > kafka_logs
+cat kafka_logs | grep OUTSIDE
+
+advertised.listeners = INSIDE://localhost:9091,OUTSIDE://localhost:32778
+listener.security.protocol.map = INSIDE:SASL_PLAINTEXT,OUTSIDE:SASL_PLAINTEXT
+listeners = INSIDE://localhost:9091,OUTSIDE://0.0.0.0:9092
+advertised.listeners = INSIDE://localhost:9091,OUTSIDE://localhost:32778
+listener.security.protocol.map = INSIDE:SASL_PLAINTEXT,OUTSIDE:SASL_PLAINTEXT
+listeners = INSIDE://localhost:9091,OUTSIDE://0.0.0.0:9092
+```
+
+So here in this example the host we should in the module's config is `localhost:32778`.
+Note that this is different between MAC and Linux machines. The above is the case for the MAC machine, and here is how 
+the respective address for a LINUX machine should look like:
+
+```console
+advertised.listeners = INSIDE://localhost:9091,OUTSIDE://172.26.0.2:9092
+listener.security.protocol.map = INSIDE:SASL_PLAINTEXT,OUTSIDE:SASL_PLAINTEXT
+listeners = INSIDE://localhost:9091,OUTSIDE://0.0.0.0:9092
+advertised.listeners = INSIDE://localhost:9091,OUTSIDE://172.26.0.2:9092
+listener.security.protocol.map = INSIDE:SASL_PLAINTEXT,OUTSIDE:SASL_PLAINTEXT
+listeners = INSIDE://localhost:9091,OUTSIDE://0.0.0.0:9092
+```
+
+So here the advertised addressed to be used in the config is `172.26.0.2:9092`.
+
+This difference comes from here: https://github.com/elastic/beats/blob/v7.5.2/libbeat/tests/compose/wrapper.go#L137
+
+This was needed before moving the metricbeat docker used in CI to host network, we can maybe remove this now if it complicates things.
+
+
+#### Configuring Kafka module
+In order to configure the Module we will use the advertised addressed to connect to the broker and the credentials
+that are also used for the tests 
+(see [test config](https://github.com/elastic/beats/blob/6c279ebf2789655725889f37820c959a8f2ea969/metricbeat/module/kafka/consumergroup/consumergroup_integration_test.go#L39)).
+Here is how the config should look like (in a MAC):
+
+```yaml
+# Kafka metrics collected using the Kafka protocol
+- module: kafka
+  metricsets:
+    - partition
+    - consumergroup
+  period: 10s
+  hosts: ["0.0.0.0:32778"]
+  username: stats
+  password: test-secret
+```
+
+
+#### Starting extra Producers/Consumers
+In order to create more stats for the Kafka Module, one could create more Producer/Consumer pairs (or combinations).
+For this we will reuse the scripts that are used withing the Docker container to bring up a Producer/Consumer pair for the testing.
+See the [source](https://github.com/elastic/beats/blob/87c49acb60b277a24c60c3956e9b4e23a644bce8/metricbeat/module/kafka/_meta/run.sh#L75).
+
+Here are the commands:
+
+```console
+{ while sleep 1; do echo message; done } | KAFKA_OPTS="-Djava.security.auth.login.config=/kafka/bin/jaas-kafka-client-producer.conf" /kafka/bin/kafka-console-producer.sh --topic test2 --broker-list localhost:9091 --producer.config /kafka/bin/sasl-producer.properties
+```
+
+Which will start a producer writing a `message` message on topic with name `test2`.
+
+```console
+KAFKA_OPTS="-Djava.security.auth.login.config=/kafka/bin/jaas-kafka-client-consumer.conf" /kafka/bin/kafka-console-consumer.sh --topic=test2 --bootstrap-server=localhost:9091 --consumer.config /kafka/bin/sasl-producer.properties
+```
+Which will start a consumer for `test2` topic.
+
+Note that starting many pairs of them(>4), it might cause the container's crash.
+
+#### JMX data
+Kafka Module also includes 3 light modules based on Jolokia Module. These are `broker`, `consumer` and `producer`.
+
+In order to explore the JMX data that are exposed by the container one can use http APIs directly like:
+
+```console
+curl -X GET http://0.0.0.0:32783/jolokia/read/kafka.server:\* | jq
+```