Allow resetting span start time

[bobrik]

This is especially useful with futures when you can create a future and a span, but only want to start the span when future is first polled.

[bobrik]

Build failed due to description being deprecated in std::error::Error.

It seems that it's recommended to use to_string() instead, but that would mean error.kind will be equal to error.message. I'm not really sure what's the best way forward here.

[sile]

Thank you for your PR!
The CI failure isn't relevant to the change introduced by you, so I'll fix it after merging this PR.

[sile]

I'm not really sure what's the best way forward here.

I think that it might be good to remove StdErrorLogFieldsBuilder::object() method and bump up the minor version. I don't want to keep using a deprecated method, but it seems meaningless to set Error::to_string() to both of error.kind and error.message field.

[sile]

Applied the above changes and published v0.4.0.