subscriber: add flatten span option to json formatter

[heat1q]

Motivation

Span fields are currently not accessible at root level but only inside a nested object or the span list. This makes indexing log data more complex if a unique span identifier is required at root level, for example in the case of distributed trace Id (See also #1531).

Solution

This change adds two new options to Json for making span fields accessible at root level:

• Json::flatten_current_span flattens all span fields into the root object. The name field is not carried over, since it might be ambiguous at root level.
• Json::flatten_span_list flattens all fields for each span in the list into the root object. Colliding fields will be overwritten from root to leaf span.

Examples

Default

{
  "timestamp": "2023-08-30T13:28:57.999070Z",
  "level": "INFO",
  "fields": {
    "message": "shaving yaks"
  },
  "target": "fmt_json::yak_shave",
  "span": {
    "yaks": 3,
    "name": "shaving_yaks"
  },
  "spans": [
    {
      "yaks": 3,
      "name": "shaving_yaks"
    }
  ]
}

Flatten only current span

All fields (except name) of the current span are present at the root level.

tracing_subscriber::fmt()
    .json()
    .flatten_current_span(true)
    .flatten_span_list(false)
    .init();

{
  "timestamp": "2023-08-30T13:30:22.753644Z",
  "level": "INFO",
  "fields": {
    "message": "shaving yaks"
  },
  "target": "fmt_json::yak_shave",
  "yaks": 3,
  "spans": [
    {
      "yaks": 3,
      "name": "shaving_yaks"
    }
  ]
}

Flatten current span and span list

tracing_subscriber::fmt()
    .json()
    .flatten_current_span(true)
    .flatten_span_list(true)
    .init();

{
  "timestamp": "2023-08-30T13:31:17.733524Z",
  "level": "INFO",
  "fields": {
    "message": "shaving yaks"
  },
  "target": "fmt_json::yak_shave",
  "yaks": 3
}

Resolves: #2670

[msabansal]

Is there any followup needed on this pr? Can we get this reviewed please.

[heat1q]

Is there any followup needed on this pr? Can we get this reviewed please.

Nothing from my side. We need to wait for the maintainers to take a look.

[coltfred]

I'd love to see this added as well. We're using spans to track ray_ids for our HTTP requests and want to be able to get rid of the span name and flatten the ray_id onto the top object.