chore(contrib/log/slog): remove singleton dependency

Fix the test failure below by removing global state from the
implementation.

```
$ go test -test.shuffle 1761195191840193457 ./contrib/log/slog/
--- FAIL: TestWrapHandler (0.03s)
    --- FAIL: TestWrapHandler/testLogger (0.03s)
        slog_test.go:103: {"time":"2025-10-23T07:14:27.195674+02:00","level":"INFO","msg":"this is an info log with tracing information","dd.trace_id":"2097471805985655682","dd.span_id":"2097471805985655682"}
        slog_test.go:103: 
                Error Trace:    /Users/felix.geisendoerfer/go/src/github.com/DataDog/dd-trace-go/contrib/log/slog/slog_test.go:42
                                                        /Users/felix.geisendoerfer/go/src/github.com/DataDog/dd-trace-go/contrib/log/slog/slog_test.go:103
                                                        /Users/felix.geisendoerfer/go/src/github.com/DataDog/dd-trace-go/contrib/log/slog/slog_test.go:149
                Error:          Not equal: 
                                expected: "68f9b9b3000000001d1bb77ea22c0b82"
                                actual  : "2097471805985655682"
                            
                                Diff:
                                --- Expected
                                +++ Actual
                                @@ -1 +1 @@
                                -68f9b9b3000000001d1bb77ea22c0b82
                                +2097471805985655682
                Test:           TestWrapHandler/testLogger
                Messages:       trace id not found
        slog_test.go:104: {"time":"2025-10-23T07:14:27.195681+02:00","level":"ERROR","msg":"this is an error log with tracing information","dd.trace_id":"2097471805985655682","dd.span_id":"2097471805985655682"}
        slog_test.go:104: 
                Error Trace:    /Users/felix.geisendoerfer/go/src/github.com/DataDog/dd-trace-go/contrib/log/slog/slog_test.go:42
                                                        /Users/felix.geisendoerfer/go/src/github.com/DataDog/dd-trace-go/contrib/log/slog/slog_test.go:104
                                                        /Users/felix.geisendoerfer/go/src/github.com/DataDog/dd-trace-go/contrib/log/slog/slog_test.go:149
                Error:          Not equal: 
                                expected: "68f9b9b3000000001d1bb77ea22c0b82"
                                actual  : "2097471805985655682"
                            
                                Diff:
                                --- Expected
                                +++ Actual
                                @@ -1 +1 @@
                                -68f9b9b3000000001d1bb77ea22c0b82
                                +2097471805985655682
                Test:           TestWrapHandler/testLogger
                Messages:       trace id not found
        slog_test.go:132: {"time":"2025-10-23T07:14:27.203777+02:00","level":"INFO","msg":"this is an info log with tracing information"}
        slog_test.go:133: {"time":"2025-10-23T07:14:27.20378+02:00","level":"ERROR","msg":"this is an error log with tracing information"}
```

Author: felixge

contrib/log/slog/slog.go

@@ -18,8 +18,6 @@ import (
 	"github.com/DataDog/dd-trace-go/v2/instrumentation/options"
 )
 
-var cfg = newConfig()
-
 func init() {
 	_ = instrumentation.Load(instrumentation.PackageLogSlog)
 }
@@ -31,16 +29,6 @@ type group struct {
 	attrs []slog.Attr
 }
 
-type config struct {
-	log128bits bool
-}
-
-func newConfig() *config {
-	return &config{
-		log128bits: options.GetBoolEnv("DD_TRACE_128_BIT_TRACEID_LOGGING_ENABLED", true),
-	}
-}
-
 // NewJSONHandler is a convenience function that returns a *slog.JSONHandler logger enhanced with
 // tracing information.
 func NewJSONHandler(w io.Writer, opts *slog.HandlerOptions) slog.Handler {
@@ -52,9 +40,18 @@ func WrapHandler(h slog.Handler) slog.Handler {
 	return &handler{wrapped: h}
 }
 
+type trilean uint8
+
+const (
+	trileanUnknown trilean = iota
+	trileanTrue
+	trileanFalse
+)
+
 type handler struct {
-	wrapped slog.Handler
-	groups  []group
+	log128BitsState trilean
+	wrapped         slog.Handler
+	groups          []group
 }
 
 // Enabled calls the wrapped handler Enabled method.
@@ -73,7 +70,7 @@ func (h *handler) Handle(ctx context.Context, rec slog.Record) error {
 	span, ok := tracer.SpanFromContext(ctx)
 	if ok && span.Context().TraceID() != tracer.TraceIDZero {
 		var traceID string
-		if cfg.log128bits {
+		if h.log128Bits() {
 			traceID = span.Context().TraceID()
 		} else {
 			traceID = strconv.FormatUint(span.Context().TraceIDLower(), 10)
@@ -101,8 +98,9 @@ func (h *handler) Handle(ctx context.Context, rec slog.Record) error {
 func (h *handler) WithAttrs(attrs []slog.Attr) slog.Handler {
 	if len(h.groups) == 0 {
 		return &handler{
-			wrapped: h.wrapped.WithAttrs(attrs),
-			groups:  h.groups,
+			wrapped:         h.wrapped.WithAttrs(attrs),
+			groups:          h.groups,
+			log128BitsState: h.log128BitsState,
 		}
 	}
 	groups := append([]group{}, h.groups...)
@@ -111,17 +109,31 @@ func (h *handler) WithAttrs(attrs []slog.Attr) slog.Handler {
 	groups[len(groups)-1] = curGroup
 
 	return &handler{
-		wrapped: h.wrapped,
-		groups:  groups,
+		wrapped:         h.wrapped,
+		groups:          groups,
+		log128BitsState: h.log128BitsState,
 	}
 }
 
 // WithGroup saves the provided group to be used later in the Handle method.
 func (h *handler) WithGroup(name string) slog.Handler {
 	return &handler{
-		wrapped: h.wrapped,
-		groups:  append(h.groups, group{name: name}),
+		wrapped:         h.wrapped,
+		groups:          append(h.groups, group{name: name}),
+		log128BitsState: h.log128BitsState,
+	}
+}
+
+// log128Bits determines the value of the log128bits field.
+func (h *handler) log128Bits() bool {
+	if h.log128BitsState == trileanUnknown {
+		if options.GetBoolEnv("DD_TRACE_128_BIT_TRACEID_LOGGING_ENABLED", true) {
+			h.log128BitsState = trileanTrue
+		} else {
+			h.log128BitsState = trileanFalse
+		}
 	}
+	return h.log128BitsState == trileanTrue
 }
 
 // IsAlreadyWrapped checks whether the given handler is already wrapped by this package.

contrib/log/slog/slog_test.go

@@ -83,8 +83,6 @@ func testLogger(t *testing.T, createLogger func(b io.Writer) *slog.Logger, asser
 	var traceID string
 	spanID := strconv.FormatUint(span.Context().SpanID(), 10)
 	if os.Getenv("DD_TRACE_128_BIT_TRACEID_LOGGING_ENABLED") == "false" {
-		// Re-initialize to account for race condition between setting env var in the test and reading it in the contrib
-		cfg = newConfig()
 		traceID = strconv.FormatUint(span.Context().TraceIDLower(), 10)
 	} else {
 		traceID = span.Context().TraceID()