A few users have expressed interest in custom metrics (whether through our metrics system or prometheus) in lambda. We should be able to do a single, synchronous collection of the metrics before flush, but it should be config-gated and not include any of the base metrics, just custom/prometheus

See meta issue and spec for the description and details:

• Meta issue: Enable custom metrics collection with AWS Lambda apm#637
• Spec issue: [META 637] Spec: Enable custom metrics collection with AWS Lambda apm#638