Switch from the Metricbeat metrics naming convention to a dedicated one

[jlvoiseux]

Motivation / Summary

Following discussions related to the addition of the Lambda metrics to the metrics-internal data stream, I has been proposed to rename the exported Lambda metrics to maintain consistency with similar metrics handled by the APM Server. The switch is as follows:

Existing

aws.lambda.metrics.billed_duration
aws.lambda.metrics.coldstart_duration
aws.lambda.metrics.duration
aws.lambda.metrics.timeout

Proposed

faas.billed_duration
faas.coldstart_duration
faas.duration
faas.timeout

Linked to elastic/apm-server#8400

How to test

Similar to #220, with this branch instead.

[apmmachine]

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• Start Time: 2022-06-17T08:24:01.084+0000

• Duration: 63 min 59 sec

Test stats 🧪

Test	Results
Failed	0
Passed	110
Skipped	2
Total	112

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