I'm writing a LoggingEnhancer for OpenCensus (census-instrumentation/opencensus-java#1212). I noticed that the Stackdriver LogEntry has a "spanId" field in https://cloud.google.com/logging/docs/reference/v2/rest/v2/LogEntry, but the google-cloud-logging LogEntry class does not (LogEntry.java). Should both LogEntry data structures have a "spanId"? If so, I can make a pull request to add the field.

/cc @g-easy