add span plugin support

ctx.go

@@ -110,6 +110,14 @@ func newSpan(ctx context.Context, f *Func, args []interface{}, trace *Trace,
 		sctx = observer.Start(sctx, s)
 	}
 
+	var finishers []SpanPluginFinisher
+	if plugins := s.f.scope.r.getSpanPlugins(); plugins != nil {
+		finishers = make([]SpanPluginFinisher, len(plugins))
+		for i, plugin := range plugins {
+			finishers[i] = plugin.Start(s)
+		}
+	}
+
 	return sctx, func(errptr *error) {
 		rec := recover()
 		panicked := rec != nil
@@ -151,6 +159,10 @@ func newSpan(ctx context.Context, f *Func, args []interface{}, trace *Trace,
 			observer.Finish(sctx, s, err, panicked, finish)
 		}
 
+		for _, finisher := range finishers {
+			finisher.Finish(err, panicked, finish)
+		}
+
 		if panicked {
 			panic(rec)
 		}

plugin.go

@@ -0,0 +1,51 @@
+// Copyright (C) 2026
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package monkit
+
+import "time"
+
+type SpanPluginFinisher interface {
+	Finish(err error, panicked bool, done time.Time)
+}
+
+type SpanPlugin interface {
+	Start(span *Span) SpanPluginFinisher
+}
+
+func (r *Registry) AddSpanPlugin(h SpanPlugin) {
+	for {
+		pluginsPtr := r.plugins.Load()
+		if pluginsPtr == nil {
+			r.plugins.CompareAndSwap(nil, new([]SpanPlugin))
+			continue
+		}
+		plugins := *pluginsPtr
+
+		nextPlugins := make([]SpanPlugin, len(plugins)+1)
+		copy(nextPlugins, plugins)
+		nextPlugins[len(plugins)] = h
+
+		if r.plugins.CompareAndSwap(pluginsPtr, &nextPlugins) {
+			return
+		}
+	}
+}
+
+func (r *Registry) getSpanPlugins() []SpanPlugin {
+	if pluginsPtr := r.plugins.Load(); pluginsPtr != nil {
+		return *pluginsPtr
+	}
+	return nil
+}

registry.go

@@ -17,6 +17,7 @@ package monkit
 import (
 	"sort"
 	"sync"
+	"sync/atomic"
 )
 
 type traceWatcherRef struct {
@@ -39,6 +40,8 @@ type registryInternal struct {
 
 	orphanMtx sync.Mutex
 	orphans   map[*Span]struct{}
+
+	plugins atomic.Pointer[[]SpanPlugin]
 }
 
 // Registry encapsulates all of the top-level state for a monitoring system.

span.go

@@ -102,6 +102,9 @@ func (s *Span) Children(cb func(s *Span)) {
 	}
 }
 
+// RawArgs returns the arguments given to the Task that created this Span.
+func (s *Span) RawArgs() []interface{} { return s.args }
+
 // Args returns the list of strings associated with the args given to the
 // Task that created this Span.
 func (s *Span) Args() (rv []string) {