[Monitoring] Convert APM-related server files that read from _source to typescript

[chrisronline]

Relates to #84370

This PR updates a couple source files used for APM monitoring that read from source to using Typescript. This is an attempt to identify all fields read from source to help with #73864

I'm going to try and do one PR for each stack product to avoid long reviews.

[elasticmachine]

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

[igoristic]

@chrisronline
Are we sure that all of these paths will always exist? When using get() we had a safeguard against errors if any of the nested path/value did not exist. Maybe we can also do it for some of these (just incase), something like:

const eventsTotalFirst = firstStats?.metrics?.libbeat?.pipeline?.events?.total ?? null;

And, maybe the types can also indicate which of these might be undefined

[chrisronline]

Excellent point @igoristic. I don't actually know what can be defined and what might be undefined. We should probably be on the safe side

[kibanamachine]

💚 Build Succeeded

• continuous-integration/kibana-ci/pull-request
• Commit: 2c6f8eb

Metrics [docs]

✅ unchanged

History

• 💚 Build #91747 succeeded a08fb5f
• 💔 Build #91529 failed d32fb96
• 💔 Build #91507 failed 2940f44

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@elasticmachine merge upstream

[igoristic]

Looks good 👍 Great job on converting these 🏆