[JSON] Extra formatter flags for compactness

[skeggse]

Feature Request

In addition to flatten_event and with_span_list for the json formatter, consider:

• flatten_spans: pull all fields of all spans up to the root json object, overriding fields hierarchically; useful to reduce unnecessary bloat in the fields for small applications that make extensive use of spans to provide context
• with_span_tail: only include the spans field when more than one span is present. when both with_current_span and with_span_list are enabled, you get the same span recorded twice, which is an eyesore and makes skimming logs more difficult.

Crates

• tracing_subscriber

Motivation

Small applications that control most of their logging/tracing spans to provide context for individual log entries can have their outputs get fairly overwhelming due to the provided json formatting capabilities.

Proposal

Main drawback is added complexity.

Alternatives

A custom formatter, maybe? I'm not sure what that would entail. Or an adapter to help reorganize the spans into a coherent json record in a different way?