Expose trace IDs in background and cloud event functions

[roffjulie]

The X-Cloud-Trace-Context header is included in requests, but as we don't expose headers to the event-style functions it is inaccessible. This value is the only means of grouping StackDriver logs by request, and so we should expose it to the user function code.

Background events: The trace ID should be available in the context parameter.

CloudEvents: The trace ID should be encoded as an extension on the event, leveraging the traceparent extension field as described in https://github.com/cloudevents/spec/blob/master/extensions/distributed-tracing.md.

[jacobg]

I'd love to have this header available in pubsub-triggered functions please!

[swa-licand]

and I'd love to have this header available in cloud storage triggered functions :)

[jaksky]

Any update on this?