change span name after it has started.

[josephhajduk]

Is your feature request related to a problem? If so, please describe it.

Attempting to write a Finagle Tracing -> Opencensus adapter. The info available at the start of the span from finagle is lacking, (i.e. just an Annotation RPC("GET") ), but as it proceeds more relevant info is revealed (uri, client, label, etc..)

Describe the solution you'd like.

A method on the Span class to change the name of the span after it has already started, but before it has ended.

Describe alternatives you've considered.

Only alternative I have considered is to bypass opencensus and try to use stackdriver api directly

Additional context.

From what I can gather this is already available in some other languages of opencensus.