We have a small set of cross cutting span attributes such as the customer's email which would benefit from being propagated to the spans of all our upstream services.
A gateway service early in the request path already adds the attribute to the span and makes further upstream requests. We can search for transactions with this span attribute from the gateway's APM page, but we'd also like to do so from upstream services. For example the billing team might want to search for requests by that customer email directly from their service's APM dashboard rather than go through the gateway's APM dashboard every time.
OpenTelemetry has the concept of baggage which can propagate key value pairs to upstream services.
Is there something similar for Elastic APM and if not what would be the best way to propagate these cross cutting span attributes?