fixed Span.Status handling

What's Changed

• Improve docs by @aldas in #8
• Added various Append* methods to Values struct to help preallocate/reuse attributes slice by @aldas in #9
• Set span status to Error when http status is >= 500 by @vadimi in #12
• Improve span status code handling and docs by @aldas in #13
• bump version to 0.0.2 by @aldas in #14

New Contributors

• @vadimi made their first contribution in #12

Full Changelog: v0.0.1...v0.0.2