Added

• Spandex.current_context/1 and Spandex.Tracer.current_context/1 functions, which get a Spandex.SpanContext struct based on the current context.
• Spandex.inject_context/3 and Spandex.Tracer.inject_context/2 functions, which inject a distributed tracing context into a list of HTTP headers.

Changed

• The Spandex.Adapter behaviour now requires an inject_context/3 callback, which encodes a Spandex.SpanContext as HTTP headers for distributed tracing.