Additional metrics based on metric filters: memory usage, etc

[CloudWatch metric filters](http://docs.aws.amazon.com/AmazonCloudWatch/latest/logs/MonitoringPolicyExamples.html) allow you to create metrics based on pattern-matching log data. This would allow us to add at least two more useful metrics:

- We can measure the memory usage of the functions (a metric which is not available by default but is logged at the end of every Lambda invocation)
- We can also count occurrences of specific error messages, e.g. "Task timed out" or "yara internal error," etc