[RFC] JSON output

[japaric]

Summary

Add a --json flag to probe-run to change the output from colored, rendered text (default) to JSON output.
The goal is to allow inter-operation with other command line tools (e.g. store in database) using JSON as an exchange format.

Detailed design: JSON schema

This Rust code:

fn function_name() {
    struct MyStruct { x: i32, y: bool }
    let x = 42;
    info!("the {} is {}", "answer", MyStruct { x, y: true });
}

Produces the following JSON object

{
  // source code location
  "location": {
    // path may be relative or absolute
    "file": "src/main.rs",
    "line": 104,
    "column": 8,
  },

  // host-side date-time at which the log was *retrieved* in RFC3339 or similar
  "host_timestamp": "2021-03-25 18:04:34+01:00",

  // target-side uptime timestamp at which the log was *emitted*
  // `null` if timestamp is omitted or custom timestamp format is used (not the `:us` hint)
  "target_timestamp": {
    "secs": 1,
    "nanos": 1234,
  },

  // Full item path to the function that contains the log statement
  // in Rust syntax: `crate_name::outer_module::inner_module::function_name`
  "path": {
      "crate": "crate_name",
      "modules": ["outer_module", "inner_module"],
      "function": "function_name",
      // check for presence of `{{impl}}` in segment retrieved from DWARF
      "is_method": false,
  }

  // 5 levels: [error, warn, info, debug, trace]
  "level": "info",

  // formatted log message
  "data": "the answer is MyStruct { x: 42, y: true }",

  // iff the log message comes from `#[panic_handler]`; otherwise `null`
  // array of stack frames
  "backtrace": [
     {
       "path": {}, // same as outer `.path` field
       "location": {}, // same as outer `.location` field
     },
     // ..
  ]
}

NOTE No attempt to include structured data in the JSON object is v1.
v2 or later may contain format string + structured data (e.g. structs becomes JSON maps, etc.)

Unresolved questions

• --output=json1 instead --json=v1?
• micros instead of nanos?