support code attributes for custom span

[wangzlei]

Description of changes:

Support code attributes for spans manually created, such as

@tracer.start_as_current_span("create-otel-span-by-decorator")
def _otel_span_decorator():
    pass

def _otel_span_api_with():
    """A demo function to illustrate OpenTelemetry span context manager."""
    with tracer.start_as_current_span("create-otel-span-by-api"):
        pass

def _otel_span_api_start_span():
    span = tracer.start_span("create-otel-span-by-api")
    span.end()

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[vastin]

LGTM