Replace metrics labels by dedicated faas fields

[jlvoiseux]

Motivation / Summary

This PR is the finishing touch to the implementation of Lambda platform metrics support. It resolves elastic/apm-server#7714 and #108.

It consists in replacing the following fields:

• labels.faas_coldstart
• labels.faas_execution
• labels.faas_id

by a faas object in the metricset generated by the Lambda extension. The following fields will then be sent to the APM Server:

• faas.coldstart
• faas.execution
• faas.id

Changes

The changes affect extension/process_metrics.go, and consist in creating the aforementioned faas object. The Go Agent APM model is also modified accordingly, as is the APM Server decoder.

How to test

• Clone this repo, checkout the current branch, build the extension and instrument a Lambda function with it.
• To start a version of the APM Server factoring in the changes of Decode Faas fields from metricset model apm-server#8373, you can use the following repository. A tool like ngrok might be useful to expose your custom APM Server to the internet.
• Set the appropriate environment variables in your test Lambda function. If you used custom-apm-server together with ngrok, only ELASTIC_APM_LAMBDA_APM_SERVER to the the address provided by ngrok.
• Validate that the following fields are part of the metrics documents:

Linked Pull Requests

elastic/apm-server#8373
elastic/apm-agent-go#1259

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• Start Time: 2022-06-16T06:35:43.344+0000

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[axw]

LGTM, just make sure to remove the "replace" before merging please.

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• Start Time: 2022-06-17T02:51:25.473+0000

• Duration: 64 min 41 sec

Test stats 🧪

Test	Results
Failed	0
Passed	110
Skipped	2
Total	112

🤖 GitHub comments

To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

• run elasticsearch-ci/docs : Re-trigger the docs validation. (use unformatted text in the comment!)