Support Datadog span error specific tags

[TAKAyukiatkwsk]

Hi 👋

I use Kamon and monitor with Datadog. In Datadog APM UI, it can show error contexts if a span has an error and specific tags. These tags are error.msg error.type error.stack.
https://docs.datadoghq.com/tracing/trace_explorer/trace_view/?tab=spantags

When we use span.fail(throwable) directly or in instruments, error.message error.stacktrace error.type tags are added to the span and send it to Datadog but it's UI doesn't show all error contexts. If we want to watch error contexts in it, I think we need to add Datadog specific tags to the span (ex. at here). What do Kamon maintainers think about it?

thanks.

[TAKAyukiatkwsk]

I created the PR #1228 about this improvement. Please review it 😄