Operation context only supports one span being set as the "current" span at a given time, and when a span ends it sets the context's current span to be its parent. In cases like Async Requests Sender where multiple requests are created, this doesn't work because the span for the first request will still exist after the span for the 2nd request has been created, but the 2nd request should be a child of the span calling ARS, not the 1st request.

Some possible ways to handle this are to (1) make a tree of telemetry contexts when entering fanout, and operate on that tree instead, or (2) reset the current span whenever entering or exiting an async handler.

There may be places other than ARS that need to follow this pattern as well.