Support AWS Lambda metrics

[simitt]

With elastic/apm-aws-lambda#108 the aws lambda extension will be sending metrics to the APM Server. Research how to handle this metrics in the first beta version in the server.

Intake API

The Intake API generically accepts any kind of metrics. Since there is no server side aggregation planned for the first version, there is no requirement for adding the specific metrics on the intake level.

Data Streams

The metrics can be considered well defined on the agent level, therefore they can be sent to the metrics-apm.internal-* data stream. Given the granularity of these metrics documents (not-aggregated, creating one event per request to the extension), we should consider storing them in a dedicated data stream though, allowing for a different retention period.

[jlvoiseux]

The following PR implements the core mechanism of encoding and forwarding Lambda Platform metrics.

Following the approval and the merge of this Pull Request, the remaining change will be to modify the APM data model to add the following fields in the metricset object:

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