[processor/deltatocumulativeprocessor] Adjust equal test to not compare functions (#38008)

#### Description

With the collector now on Go 1.23 I'm working on adding iterator support
for pdata at
open-telemetry/opentelemetry-collector#12380 but
the EqualT_test fails when comparing the iterators returned by All.

Since the reflect value of an iter.Seq/iter.Seq2 is reflect.Func, they
are not comparable in the usual sense. The go-cmp/cmp is still on Go
1.21 so it does not know about iterators. There is `x/exp/xiter.Equal`
but it is still an open proposal.
See
https://go.googlesource.com/go/+/81c66e71d480ae2372b7eea4bcdf600b50fdd5e1/src/reflect/deepequal.go#158.

Examples of the test failures:

```txt
expo_test.go:203: Attributes().All(): 0xd1b400 != 0xd1b400
expo_test.go:203: Exemplars().All(): 0xe2be20 != 0xe2be20
```

This PR adjusts the equal logic to skip the equality check for
functions.

#### Link to tracking issue
For open-telemetry/opentelemetry-collector#12380

Author: mauri870

processor/deltatocumulativeprocessor/internal/data/datatest/equal.go

@@ -31,6 +31,8 @@ func New(tb testing.TB) T {
 //   - exactly zero input arguments
 //   - exactly one return value
 //   - does not start with 'Append'
+//   - does not start with 'Clone'
+//   - does not return a function
 //
 // If this yields differences, those are reported and the test fails.
 // If the compared values are [pmetric.ExponentialHistogramDataPoint], then
@@ -78,6 +80,13 @@ func equal(tb testing.TB, want, got any, name string) bool {
 		if mw.Type().NumIn() != 0 || mw.Type().NumOut() != 1 {
 			continue
 		}
+
+		// skip equality check for methods returning functions
+		ret := mw.Type().Out(0)
+		if ret.Kind() == reflect.Func {
+			continue
+		}
+
 		// Append(Empty) fails above heuristic, exclude it
 		if strings.HasPrefix(mname, "Append") || strings.HasPrefix(mname, "Clone") {
 			continue

processor/deltatocumulativeprocessor/internal/data/datatest/equal_test.go

@@ -5,6 +5,7 @@ package datatest
 
 import (
 	"fmt"
+	"iter"
 	"path/filepath"
 	"runtime"
 	"strconv"
@@ -35,11 +36,43 @@ func ExampleT_Equal() {
 	is.Equal(want, got)
 
 	// Output:
-	// equal_test.go:35: Negative().BucketCounts().AsRaw(): [1 1 2] != [4]
-	// equal_test.go:35: Negative().BucketCounts().Len(): 3 != 1
-	// equal_test.go:35: Positive().BucketCounts().AsRaw(): [1 1 2] != [4]
-	// equal_test.go:35: Positive().BucketCounts().Len(): 3 != 1
-	// equal_test.go:35: Scale(): 0 != 1
+	// equal_test.go:36: Negative().BucketCounts().AsRaw(): [1 1 2] != [4]
+	// equal_test.go:36: Negative().BucketCounts().Len(): 3 != 1
+	// equal_test.go:36: Positive().BucketCounts().AsRaw(): [1 1 2] != [4]
+	// equal_test.go:36: Positive().BucketCounts().Len(): 3 != 1
+	// equal_test.go:36: Scale(): 0 != 1
+}
+
+type structFunc struct {
+	a int
+}
+
+func (s structFunc) Get() int {
+	return s.a
+}
+
+func (s structFunc) Func() func() {
+	return func() {}
+}
+
+// iter.Seq is a reflect.Func
+func (s structFunc) Seq() iter.Seq[int] {
+	return func(_ func(v int) bool) {
+	}
+}
+
+// iter.Seq2 is a reflect.Func
+func (s structFunc) Seq2() iter.Seq2[int, string] {
+	return func(_ func(k int, v string) bool) {
+	}
+}
+
+func TestEqualMethodIgnoreFuncReturnType(t *testing.T) {
+	is := datatest.New(t)
+	s := structFunc{a: 42}
+	want := any(s)
+	got := any(s)
+	is.Equal(want, got)
 }
 
 func TestNone(*testing.T) {}