subscriber: use display_timestamp and display_level in `Json::for…

…mat_event` (#1463)

## Motivation

It should be possible to remove `timestamp` and `level` fields when
using json formatting.

As of now this has no effect:

```rs
let subscriber = tracing_subscriber::fmt()
    .with_level(false)
    .without_time()
    .json()
    .finish();
```

## Solution

Use the existing `display_timestamp` and `display_level` fields to
conditionally serialize `timestamp` and `level`.

tracing-subscriber/src/fmt/format/json.rs

@@ -207,8 +207,13 @@ where
 
             let mut serializer = serializer.serialize_map(None)?;
 
-            serializer.serialize_entry("timestamp", &timestamp)?;
-            serializer.serialize_entry("level", &meta.level().as_serde())?;
+            if self.display_timestamp {
+                serializer.serialize_entry("timestamp", &timestamp)?;
+            }
+
+            if self.display_level {
+                serializer.serialize_entry("level", &meta.level().as_serde())?;
+            }
 
             let format_field_marker: std::marker::PhantomData<N> = std::marker::PhantomData;