subscriber: allow to customize Registry with sharded_slab Config.

Adds CollectionBuilder with_slab_config::<S>() func to this end. This e.g.
allows to use tracing in environment with >= 4k threads.

c.f. tokio-rs#1485

tracing-subscriber/src/fmt/fmt_subscriber.rs

@@ -654,6 +654,19 @@ impl<C, N, E, W> Subscriber<C, N, E, W> {
             _inner: self._inner,
         }
     }
+
+    /// Updates the collector type.
+    pub fn set_collector<C2>(self) -> Subscriber<C2, N, E, W> {
+        Subscriber {
+            fmt_event: self.fmt_event,
+            fmt_fields: self.fmt_fields,
+            fmt_span: self.fmt_span,
+            make_writer: self.make_writer,
+            is_ansi: self.is_ansi,
+            log_internal_errors: self.log_internal_errors,
+            _inner: PhantomData,
+        }
+    }
 }
 
 impl<C> Default for Subscriber<C> {

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

@@ -507,7 +507,10 @@ impl<'a> field::Visit for JsonVisitor<'a> {
 #[cfg(test)]
 mod test {
     use super::*;
-    use crate::fmt::{format::FmtSpan, test::MockMakeWriter, time::FormatTime, CollectorBuilder};
+    use crate::fmt::{
+        format::FmtSpan, test::MockMakeWriter, time::FormatTime, CollectorBuilder,
+        DefaultSlabConfig,
+    };
 
     use tracing::{self, collect::with_default};
 
@@ -521,7 +524,7 @@ mod test {
         }
     }
 
-    fn collector() -> CollectorBuilder<JsonFields, Format<Json>> {
+    fn collector() -> CollectorBuilder<DefaultSlabConfig, JsonFields, Format<Json>> {
         crate::fmt::CollectorBuilder::default().json()
     }
 
@@ -795,7 +798,7 @@ mod test {
 
     fn test_json<T>(
         expected: &str,
-        builder: crate::fmt::CollectorBuilder<JsonFields, Format<Json>>,
+        builder: crate::fmt::CollectorBuilder<DefaultSlabConfig, JsonFields, Format<Json>>,
         producer: impl FnOnce() -> T,
     ) {
         let make_writer = MockMakeWriter::default();
@@ -817,7 +820,7 @@ mod test {
 
     fn test_json_with_line_number<T>(
         expected: &str,
-        builder: crate::fmt::CollectorBuilder<JsonFields, Format<Json>>,
+        builder: crate::fmt::CollectorBuilder<DefaultSlabConfig, JsonFields, Format<Json>>,
         producer: impl FnOnce() -> T,
     ) {
         let make_writer = MockMakeWriter::default();