Convention for cancelled spans

[charleskorn]

It would be great if there was a language- and protocol-agnostic convention for marking a span as representing a cancelled operation.

At present, different languages and protocols treat cancelled operations in different ways, which makes it difficult to do things like filter a trace to show only non-cancelled operations, or visually distinguish between successful, failed and cancelled operations.

For example, a span representing a gRPC call would have the rpc.grpc.status_code attribute set to 1 (for the gRPC CANCELLED status code), while a span representing a Golang operation might be recorded with an event with a context canceled message.

Instead, it'd be great if there was a convention for an attribute that marked a span as cancelled, such as cancelled.