Downloading CloudWatch metrics

[mcopik]

Currently, we query CloudWatch metrics to get all execution statistics. However, this struggles to return all results if we do many concurrent invocations - we get 90-95% invocations instead. We want to investigate the source of the issue.

Alternatively, we can try a different method that works and always returns 100% of invocations - export CloudWatch metrics to S3, download them and parse manually. We have code for that in a different project.

This can be related to issue #42