core: preserve metrics registry in Context.WithValue (#7861)

* core: preserve metrics registry in Context.WithValue

Context.WithValue rebuilt the Context without copying the unexported
metricsRegistry field, so any module provisioned under a context
derived via WithValue saw a nil registry from GetMetricsRegistry().

* Potential fix for pull request finding

Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>
Signed-off-by: Kévin Dunglas <kevin@dunglas.fr>

---------

Signed-off-by: Kévin Dunglas <kevin@dunglas.fr>
Co-authored-by: Copilot Autofix powered by AI <175728472+Copilot@users.noreply.github.com>

context.go

@@ -676,6 +676,7 @@ func (ctx *Context) WithValue(key, value any) Context {
 		ancestry:        ctx.ancestry,
 		cleanupFuncs:    ctx.cleanupFuncs,
 		exitFuncs:       ctx.exitFuncs,
+		metricsRegistry: ctx.metricsRegistry,
 	}
 }
 

context_test.go

@@ -15,10 +15,28 @@
 package caddy
 
 import (
+	"context"
 	"encoding/json"
 	"io"
+	"testing"
 )
 
+func TestContextWithValuePreservesMetricsRegistry(t *testing.T) {
+	ctx, cancel := NewContext(Context{Context: context.Background()})
+	t.Cleanup(cancel)
+
+	reg := ctx.GetMetricsRegistry()
+	if reg == nil {
+		t.Fatal("expected a metrics registry on the base context")
+	}
+
+	type testKey struct{}
+	derived := ctx.WithValue(testKey{}, "value")
+	if got := derived.GetMetricsRegistry(); got != reg {
+		t.Fatalf("WithValue must preserve the metrics registry: got %p, want %p", got, reg)
+	}
+}
+
 func ExampleContext_LoadModule() {
 	// this whole first part is just setting up for the example;
 	// note the struct tags - very important; we specify inline_key