Enable custom metrics collection with AWS Lambda

# Description
Currently metrics collection is disabled on APM agents when they run on AWS Lambda.
Reporting periodically collected metrics ( CPU usage, etc. ) requires a proper strategy on the Lambda extension or the APM server and will be handled in a separate issue.

With this issue, we want to enable collection of custom metrics (e.g. counters collected through corresponding metrics libraries, such as Micrometer, Prometheus, Node Agent's metrics API). Custom metrics should be flushed at the end of a Lambda execution in the same way it's currently done with the trace data.

## Spec Issue
- [ ] https://github.com/elastic/apm/issues/638

## Agent Issues
- [ ] https://github.com/elastic/apm-agent-java/issues/2592
- [ ] https://github.com/elastic/apm-agent-nodejs/issues/2659
- [ ] https://github.com/elastic/apm-agent-python/issues/1534