Expose trace IDs in 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.

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.

[grayside]

General discussion about exposing HTTP headers in GoogleCloudPlatform/functions-framework#34

[josephlewis42]

Closing this in favor of the GoogleCloudPlatform/functions-framework#34 issue which we'll use to standardize behavior across languages.