Possible log field collision when used with `tracing-actix-web`

Nobody has claimed this yet.

This issue takes over from https://github.com/LukeMathWalker/tracing-actix-web/issues/1

When we define a tracing instrument that has fields also present in the `info_span!` used in [`tracing-actix-web`](https://github.com/LukeMathWalker/tracing-actix-web), the collision results in the `tracing-actix-web` fields getting silently overwritten when logged out by this crate (potentially by other logs too, I'm not too sure!).

Here is the `info_span!` from [`tracing-actix-web`](https://github.com/LukeMathWalker/tracing-actix-web/blob/master/src/lib.rs#L136):

```rust
let span = tracing::info_span!(
"Request",
request_path = %req.path(),
user_agent = %user_agent,
client_ip_address = %req.connection_info().realip_remote_addr().unwrap_or(""),
request_id = %Uuid::new_v4(),
status_code = tracing::field::Empty,
);
```

### Example

If we e.g. use the following macro in our code:

```rust
#[tracing::instrument(
name = "Adding a new subscriber",
skip(payload, pool),
fields(
request_id=%Uuid::new_v4(),
email = %payload.email,
name = %payload.name
)
)]
```

We'll have a collision on `request_id`, and we'll only see the `request_id` defined in our instrument in the resulting logs, and not the one from `tracing-actix-web`. This means we can potentially lose visibility in our logs (or just make it a bit more complex).

### Expected Behaviour

As discussed in https://github.com/LukeMathWalker/tracing-actix-web/issues/1, a good outcome might be for this crate to throw a warning on such a collision.

### Thanks

For the excellent crate!

Start with the linked tracing-actix-web src/lib.rs info_span! definition and trace how tracing-bunyan-formatter processes span fields. Investigate what happens when an instrument field such as request_id collides with an existing field. Done should include a warning for the collision while retaining clear visibility of the relevant fields in formatted logs.

Written by the indexing model from the issue text.