Per function and per function call logs via API

[treeder]

Currently logs are emitted for each function call with the following values:

call_id="123"
function="FUNCTION_NAME"

If it's the user's logs (written to STDERR), there is also:

user_log=true

We should have some default way of storing and retrieving the logs via the API so the out of the box experience is nice. This probably means we may have an endpoint, but the implementation of that endpoint is pluggable.

[pedronasser]

@treeder Any idea of how we are going to store those logs?

[vadan-cs]

Are there plans to add an HTTP endpoint to https://github.com/iron-io/functions/blob/master/api/server/server.go to retrieve function logs?