subscriber, serde: add valuable support to the JSON formatter

[hawkw]

This branch introduces support for valuable in tracing-subscriber's
JSON formatter, and in tracing-serde, using the valuable-serde
bridge.

This allows the fmt::Json subscriber to actually record valuable
values as structured JSON. Here's an example, where a field is first
recorded using fmt::Debug, and then again using field::valuable:

:; RUSTFLAGS="--cfg tracing_unstable" cargo run --example valuable_json | jq
    Finished dev [unoptimized + debuginfo] target(s) in 0.09s
     Running `target/debug/examples/valuable_json`
{
  "timestamp": "2022-01-25T21:36:30.729658Z",
  "level": "INFO",
  "fields": {
    "valuable": false,
    "user": "User { name: \"Arwen Undomiel\", age: 3000, address: Address { country: \"Middle Earth\", city: \"Rivendell\", street: \"leafy lane\" } }"
  },
  "target": "valuable_json"
}
{
  "timestamp": "2022-01-25T21:36:30.729720Z",
  "level": "INFO",
  "fields": {
    "valuable": true,
    "user": {
      "name": "Arwen Undomiel",
      "age": 3000,
      "address": {
        "country": "Middle Earth",
        "city": "Rivendell",
        "street": "leafy lane"
      }
    }
  },
  "target": "valuable_json"
}

Note that this is currently not mergeable, as it adds a dependency on
the valuable-serde crate, which is unreleased. Also, I'm not sure if
the feature flagging is quite correct in a couple places.

[hawkw]

As a side note, this branch also nicely validates that recording Valuable
values from a subscriber is actually possible, which the previous valuable PR
didn't have an example of. I am a bit unsure if it's actually the best thin for
record_value to take &dyn Valuable, rather than just taking a Value. In
practice, basically all that you can do with the Valuable trait object is call
as_value on it, anyway. @carllerche any advice?

[hawkw]

depends on tokio-rs/valuable#82

[davidbarsky]

I wanted to maybe flatten the JSON output, but there isn't any API space to do that. Next version of subscriber, I guess.

[hawkw]

I wanted to maybe flatten the JSON output, but there isn't any API space to do that. Next version of subscriber, I guess.

Sorry, can you explain what you mean by this? this API already exists, and I believe it will flatten fields into the top-level object...this change doesn't affect that.