Function logs can be reported without faas.execution field set

[lahsivjar]

APM AWS Lambda extension version (main):

Describe the bug
It is possible that function logs are reported without the faas.execution field set since the current code assumes that both platform.start and function logs for a given invocation will be reported in the same batch (relevant code).

To Reproduce
Steps to reproduce the behavior:

1. Run a lambda function generating function logs with the latest version of extension (built from main).
2. Invoke the function a bunch of times.
3. Observe the missing faas.execution field on Kibana for some logs.

Expected behavior
All logs should have faas.execution set. This being said, it is not always possible to have faas.execution especially if the platform.start and function logs are delivered to two separate instances which I think should be possible.

Provide logs (if relevant):

^ Snapshot showing missing faas.execution in logs.