metrics: revert changes to MultiLabelMap's String method

This breaks its ability to be used as an expvar and is blocking a trunkd
deploy. Revert for now, and add a test to ensure that we don't break it
in a future change.

Updates tailscale#13550

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: I1f1221c257c1de47b4bff0597c12f8530736116d

metrics/multilabelmap.go

@@ -97,12 +97,8 @@ type KeyValue[T comparable] struct {
 }
 
 func (v *MultiLabelMap[T]) String() string {
-	var sb strings.Builder
-	sb.WriteString("MultiLabelMap:\n")
-	v.Do(func(kv KeyValue[T]) {
-		fmt.Fprintf(&sb, "\t%v: %v\n", kv.Key, kv.Value)
-	})
-	return sb.String()
+	// NOTE: This has to be valid JSON because it's used by expvar.
+	return `"MultiLabelMap"`
 }
 
 // WritePrometheus writes v to w in Prometheus exposition format.

metrics/multilabelmap_test.go

@@ -5,6 +5,7 @@ package metrics
 
 import (
 	"bytes"
+	"encoding/json"
 	"expvar"
 	"fmt"
 	"io"
@@ -129,3 +130,21 @@ func BenchmarkMultiLabelWriteAllocs(b *testing.B) {
 		m.WritePrometheus(w, "test")
 	}
 }
+
+func TestMultiLabelMapExpvar(t *testing.T) {
+	m := new(MultiLabelMap[L2])
+	m.Add(L2{"a", "b"}, 2)
+	m.Add(L2{"b", "c"}, 4)
+
+	em := new(expvar.Map)
+	em.Set("multi", m)
+
+	// Ensure that the String method is valid JSON to ensure that it can be
+	// used by expvar.
+	encoded := []byte(em.String())
+	if !json.Valid(encoded) {
+		t.Fatalf("invalid JSON: %s", encoded)
+	}
+
+	t.Logf("em = %+v", em)
+}