v3.1.0

What's Changed

Encode logger metadata as string. by @aselder in https://github.com/spandex-project/spandex/pull/127
Set up sponsorship links by @GregMefford in https://github.com/spandex-project/spandex/pull/132
Guard clauses for trace and span macros by @GregMefford in https://github.com/spandex-project/spandex/pull/130
Misc doc changes by @kianmeng in https://github.com/spandex-project/spandex/pull/128

@aselder made their first contribution in https://github.com/spandex-project/spandex/pull/127

Full Changelog: https://github.com/spandex-project/spandex/compare/3.0.3...3.1.0