tracing: fix spawn_blocking location fields

In a previous PR (#4128), the `spawn.location` field on task spans was
structured into 3 separate fields for the `file`, `line`, and `col`.
There is a separately created span for blocking tasks which was missed.

This caused tasks created with `spawn_blocking` to appear in
`tokio-console` without a location, but with an additional "free form"
field containing the formatted source code location.

This change modifies this span to use the same format. The span creation
needs to be separate from the other task spans because it records the
function name. This information is useful in the `spawn_blocking` case,
but can be "catastrophically long" in the `async fn` case and was
removed in #3074.

tokio/src/runtime/blocking/pool.rs

@@ -371,7 +371,9 @@ impl Spawner {
                 task.name = %name.unwrap_or_default(),
                 task.id = id.as_u64(),
                 "fn" = %std::any::type_name::<F>(),
-                spawn.location = %format_args!("{}:{}:{}", location.file(), location.line(), location.column()),
+                loc.file = location.file(),
+                loc.line = location.line(),
+                loc.col = location.column(),
             );
             fut.instrument(span)
         };