add support for emitting span events

When emitted, the span events look like this (for an imaginary function
called `print`, taking two arguments):

    ts=2025-08-29T13:29:27.488649565Z level=info target=_rsplayground span=print span_path=print message=new
    ts=2025-08-29T13:29:27.488658128Z level=info target=_rsplayground span=print span_path=print message=enter arg1="hello" arg2="world"
    ts=2025-08-29T13:29:27.488678624Z level=info target=_rsplayground span=print span_path=print message=exit
    ts=2025-08-29T13:29:27.488694014Z level=info target=_rsplayground span=print span_path=print message=close time.busy=20.5µs time.idle=23.7µs

Author: Leandros

src/formatter/builder.rs

@@ -1,6 +1,6 @@
 use tracing::Subscriber;
 use tracing_subscriber::{
-    fmt::{Layer, SubscriberBuilder},
+    fmt::{format::FmtSpan, Layer, SubscriberBuilder},
     registry::LookupSpan,
 };
 
@@ -9,6 +9,7 @@ use crate::{EventsFormatter, FieldsFormatter};
 pub struct Builder {
     events: EventsFormatter,
     fields: FieldsFormatter,
+    span_events: FmtSpan,
 }
 
 /// Create a builder that can be used to configure the formatter.
@@ -34,6 +35,7 @@ impl Builder {
         Self {
             events: EventsFormatter::default(),
             fields: FieldsFormatter::default(),
+            span_events: FmtSpan::NONE,
         }
     }
 
@@ -53,6 +55,10 @@ impl Builder {
         self.events.with_span_path = enable;
         self
     }
+    pub fn with_span_events(mut self, kind: FmtSpan) -> Self {
+        self.span_events = kind;
+        self
+    }
     pub fn with_location(mut self, enable: bool) -> Self {
         self.events.with_location = enable;
         self
@@ -76,6 +82,7 @@ impl Builder {
         S: Subscriber + for<'a> LookupSpan<'a>,
     {
         tracing_subscriber::fmt::layer()
+            .with_span_events(self.span_events)
             .event_format(self.events)
             .fmt_fields(self.fields)
     }

src/lib.rs

@@ -20,3 +20,4 @@ mod serializer;
 
 pub use crate::formatter::builder::{builder, Builder};
 pub use crate::formatter::{layer, EventsFormatter, FieldsFormatter};
+pub use tracing_subscriber::fmt::format::FmtSpan;