Configure Logstash output has an non-working example due to a constant_keyword field

[marclop]

Description

One of the examples in configuring a Logstash output with APM Server is broken. The issue is that data_stream.dataset is a constant_keyword in the shipped mappings and it will cause the example to not work.

We should update the example to point out that due to the shipped mappings this example will also need a @custom component template to override that field's mapping from constant_keyword to keyword.