subscriber: record error source with Json format

[BugenZhao]

Feature Request

Crates

tracing-subscriber

Motivation

There's no strong convention for how an Error should display itself. However, an increasing number of crates no longer include the error source in their Display implementation any more (aws-sdk for example: awslabs/aws-sdk-rust#657). So it becomes the callers' or the loggers' responsibility to visit the source chain and get them formatted.

We already done this in the DefaultVisitor and PrettyVisitor for the fmt layer. And I'm wondering whether we should also handle the sources in JsonVisitor as well.

Proposal

Override Visit::record_error in JsonVisitor, add additional field named <field>.sources with type string[] for error sources (if exist).

Alternatives

[SuperFluffy]

I was also caught by this recently changing from the tracing subscriber pretty-printer to json-formatter.

I am investigating alternatives to fmt::Subscriber, but it would be nice if it picked up the error sources as well.

[evbo]

can anyone share how you currently use tracing::error! in an AWS lambda in order for it to log error messages correctly?