SpanBuilderShim: set span attribute before starting a span

[shawnsong]

Is your feature request related to a problem? Please describe.
In OpenTracing, we can elevate a trace's importance by adding a sampling.priority tag to a span. This doesn't seem to be supported in the opentracing-shim library. SpanBuilderShim starts a Span before its attributes are copied. This would cause the sampling decision to be made without considering the value of sampling.priority.

Describe the solution you'd like
Is it possible to copy the attributes/tags before starting a Span so we can have our own Sampler implementation which can respect the value of sampling.priority?

...

for (int i = 0; i < this.spanBuilderAttributeKeys.size(); i++) {
  AttributeKey key = this.spanBuilderAttributeKeys.get(i);
  Object value = this.spanBuilderAttributeValues.get(i);
  builder.setAttribute(key, value);
}

io.opentelemetry.api.trace.Span span = builder.startSpan();

if (error) {
  span.setStatus(StatusCode.ERROR);
}
...