[Stack Monitoring] Document data stored for Logstash module

[jasonrhodes]

For the kibana-xpack Metricbeat module, answer the following questions in a Markdown file:

• What endpoints does Metricbeat request data from, and what data is requested?
• Of the data we request, what gets stored in the ES indices? Do we have a description of each field anywhere? What is the source of truth?

Stretch goals (if these become too difficult to answer, we can separate to a new ticket):

• Of the documents and fields that we store, how many are queried by the Kibana app? How many are never queried?
• Of the documents and fields queried, which are used for which graphs and charts and tables etc in the UI?

Notes:

• Consider using monitoring.ui.debug_mode kibana.yml option to visualize queries being made per page

[elasticmachine]

Pinging @elastic/infra-monitoring-ui (Team:Infra Monitoring UI)