subscriber: add space when recording new values on an existing span

## Motivation 

Fix the issue #627 

## Solution 

If the remaining values is not empty I just append a whitespace to
format.

BTW than you all for your work and tag `Good first issue` ❤️ you build 
a real opensource project with a real community.

Fixes #627 

Signed-off-by: Benjamin Coenen <[email protected]>

tracing-subscriber/src/fmt/fmt_layer.rs

@@ -445,6 +445,9 @@ where
         if let Some(FormattedFields { ref mut fields, .. }) =
             extensions.get_mut::<FormattedFields<N>>()
         {
+            if !values.is_empty() {
+                fields.push(' ');
+            }
             let _ = self.fmt_fields.format_fields(fields, values);
         } else {
             let mut buf = String::new();