subscriber: record error sources in fmt layer with JSON format

[BugenZhao]

Motivation

Hi, all. As proposed in #2784, this PR records the error sources in fmt layer with JSON format, for both spans and events.

Solution

• Override record_error of fmt::JsonVisitor for span fields.
• Override record_error of serde::SerdeMapVisitor for event fields.
• Use a new field named {field}.sources to record the sources as a list.
• Add a unit test.

[stijnherfst]

It would be great to have this feature in.

[LucioFranco]

Closing this one in favor of #3317