exporters: support MAP attributes (#8453)

Fixes https://github.com/open-telemetry/opentelemetry-go/issues/8163

Add end-to-end exporter handling for `attribute.SLICE` in the remaining
paths that still treated it as invalid or relied on fallback formatting.

Changes:

- encode `attribute.MAP` as OTLP `AnyValue_AnyValue_KvlistValue` for
trace, log, and metric transforms
- serialize Zipkin `MAP` attributes using the non-OTLP AnyValue string
representation

exporters/zipkin/model.go

@@ -182,10 +182,10 @@ func attributeToStringPair(kv attribute.KeyValue) (string, string) {
 		}
 		encoded, _ := json.Marshal(data)
 		return string(kv.Key), string(encoded)
-	case attribute.SLICE:
+	case attribute.SLICE, attribute.MAP:
 		// Note that this is a best effort support as this exporter is already deprecated.
 		// Yet, we want to preserve other existing behavior as much as possible.
-		// Emit the slice using the non-OTLP AnyValue string representation.
+		// Emit the value using the non-OTLP AnyValue string representation.
 		// Some values will be emitted differently than the specific types above.
 		return string(kv.Key), kv.Value.String()
 	default:

exporters/zipkin/model_test.go

@@ -1003,6 +1003,29 @@ func TestAttributeToStringPairSlice(t *testing.T) {
 	assert.Equal(t, `[true,"Ymlu",[2,null]]`, v)
 }
 
+func TestAttributeToStringPairMap(t *testing.T) {
+	k, v := attributeToStringPair(attribute.Map(
+		"map",
+		attribute.String("string", "o"),
+		attribute.Int("number", 2),
+		attribute.ByteSlice("bytes", []byte("bin")),
+		attribute.Slice(
+			"slice",
+			attribute.BoolValue(true),
+			attribute.MapValue(attribute.String("inner", "value")),
+		),
+		attribute.Map("nested", attribute.Bool("ok", true)),
+		attribute.KeyValue{Key: "empty"},
+	))
+
+	assert.Equal(t, "map", k)
+	assert.JSONEq(
+		t,
+		`{"bytes":"Ymlu","empty":null,"nested":{"ok":true},"number":2,"slice":[true,{"inner":"value"}],"string":"o"}`,
+		v,
+	)
+}
+
 func zkmodelIDPtr(n uint64) *zkmodel.ID {
 	id := zkmodel.ID(n)
 	return &id