[META] [Monitoring] Identify all values used from _source

[chrisronline]

To help with #73864, we need to help the Beats team identify the fields that need aliasing (which is something we've done already with #73864 (comment)) AND the fields read directly from _source. The effort do to this is great, as we don't have any way to automatically identify these without manually combing through source files.

I think the best way to handle this to convert the files that do read from _source into *.ts files. Then, create a global interface that describes all the fields read with the appropriate types. This will take some time too, but it should also help move us forward with our migration to *.ts

I'm planning to separate this work by stack product

• APM -> [Monitoring] Convert APM-related server files that read from _source to typescript #84829
• Beats -> [Monitoring] Convert Beats-related server files that read from _source to typescript #85193
• Kibana -> [Monitoring] Convert Kibana-related server files that read from _source to typescript #86364
• Elasticsearch -> [Monitoring] Convert Elasticsearch-related server files that read from _source to typescript #88212
• Logstash -> [Monitoring] Convert Logstash-related server files that read from _source to typescript #86787

[elasticmachine]

Pinging @elastic/stack-monitoring (Team:Monitoring)

[chrisronline]

@igoristic What do you think of this approach? I did one file here: #84375

[sgrodzicki]

@chrisronline should we close this as all listed PRs are merged?

[chrisronline]

We can close it, but I will also use this ticket to communicate the list of fields to @sayden and @ycombinator