Add the Outcome field to Transactions and Spans.

If when a transaction or span is ended, its outcome has not been specified, we either use some protocol/instrumentation-specific logic to set a default, or otherwise set it to "unknown". For HTTP and gRPC, we use the status code to set the outcome.

• HTTP server side: "success" for status codes < 500, "failure" otherwise.
• HTTP client side: "success" for status codes < 400, "failure" otherwise.
• gRPC server side:
  • "unknown" for status code Unknown
  • "failure" for status codes DataLoss, Unavailable, Internal, and Unimplemented
  • "success" otherwise
• gRPC client side:
  • "unknown" for status code Unknown
  • "success" for status code OK
  • "failure" otherwise

Closes #799