This is a partial revert of b3f5eeb3. While
using a type switch is more efficient, it also differentiates between a
`string` type and a derived type, such as `ext.SpanTypeEnum`. By reverting back
to using reflect for the type match, we fix this regression.

This change keeps the `fmt.Stringer` interface match, which is used for
`*url.URL`.

A test case for `ext.SpanTypeEnum` has been added.

fix(tracing): Add back support for formatting derived scalar types.

See merge request !237



Merged-by: Igor <iwiedler@gitlab.com>
Approved-by: Igor <iwiedler@gitlab.com>
Co-authored-by: Florian Forster <fforster@gitlab.com>