Add span tags dynamically

[0xd34d10cc]

tracing::instrument supports following use case:

// This function does a very interesting and important mathematical calculation.
// Suppose we want to record both the inputs to the calculation *and* its result...
#[instrument(fields(result))]
pub fn do_calculation(input_1: usize, input_2: usize) -> usize {
    // Rerform the calculation.
    let result = input_1 + input_2;

    // Record the result as part of the current span.
    tracing::Span::current().record("result", &result);

    // ... etc ...
}

Is it possible to implement similar feature in minitrace?