Add metrics for external memory

[SimonSimCity]

This ticket adds some metrics missing of #309.

I've added the metrics nodejs.memory.external.bytes and nodejs.memory.arrayBuffers.bytes (available from node 13.x).

All memory related data is now collected using process.memoryUsage() which has a better documentation than the internal v8 methods: https://nodejs.org/docs/latest/api/process.html#process_process_memoryusage

Checklist

• Implement code
• Add tests
• Update documentation
• Commit message follows commit guidelines

[cla-checker-service]

💚 CLA has been signed

[bmorelli25]

@elasticmachine, run elasticsearch-ci/docs

[SimonSimCity]

Anything postponing this? I'd like to have it in soon to monitor long-term memory development of the C-libraries of my node-application.

[lreuven]

Hi,
Thanks for the contribution,
@vigneshshanmugam can you have a look when you have the time?

[ghost]

💚 Build Succeeded

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Build stats

• Build Cause: [Started by user Vignesh Shanmugam, Replayed Add simple Elasticsearch instrumentation #10]

• Start Time: 2020-05-04T12:07:02.210+0000

• Duration: 19 min 26 sec (1166112)

• Commit: 582cb6f

Test stats 🧪

Test	Results
Failed	0
Passed	143192
Skipped	0
Total	143192

[vigneshshanmugam]

Thanks @SimonSimCity, the code looks good to me

@axw One question on the APM Server part. Do we control the custom metrics in any way or we can keep adding new ones whenever we find the use-case for it?

[bmorelli25]

@elasticmachine, run elasticsearch-ci/docs

[axw]

One question on the APM Server part. Do we control the custom metrics in any way or we can keep adding new ones whenever we find the use-case for it?

@vigneshshanmugam as long as there's a clear use-case for it, it's fine to add new metrics. It's best to align across agents where we can, but these ones are pretty Node.js-specific so no worries there.