Add ByteSize method for Labels (#16717)

Add `ByteSize()` method to different labels implementations.
One of the use case so that we can track the memory used by Labels.

Signed-off-by: Jon Kartago Lamida <me@lamida.net>

model/labels/labels.go

@@ -248,6 +248,17 @@ func (ls Labels) WithoutEmpty() Labels {
 	return ls
 }
 
+// ByteSize returns the approximate size of the labels in bytes including
+// the two string headers size for name and value.
+// Slice header size is ignored because it should be amortized to zero.
+func (ls Labels) ByteSize() uint64 {
+	var size uint64 = 0
+	for _, l := range ls {
+		size += uint64(len(l.Name)+len(l.Value)) + 2*uint64(unsafe.Sizeof(""))
+	}
+	return size
+}
+
 // Equal returns whether the two label sets are equal.
 func Equal(ls, o Labels) bool {
 	return slices.Equal(ls, o)

model/labels/labels_dedupelabels.go

@@ -417,6 +417,13 @@ func (ls Labels) WithoutEmpty() Labels {
 	return ls
 }
 
+// ByteSize returns the approximate size of the labels in bytes.
+// String header size is ignored because it should be amortized to zero.
+// SymbolTable size is also not taken into account.
+func (ls Labels) ByteSize() uint64 {
+	return uint64(len(ls.data))
+}
+
 // Equal returns whether the two label sets are equal.
 func Equal(a, b Labels) bool {
 	if a.syms == b.syms {

model/labels/labels_dedupelabels_test.go

@@ -30,6 +30,15 @@ var expectedSizeOfLabels = []uint64{ // Values must line up with testCaseLabels.
 	325,
 }
 
+var expectedByteSize = []uint64{ // Values must line up with testCaseLabels.
+	8,
+	0,
+	8,
+	8,
+	8,
+	32,
+}
+
 func TestVarint(t *testing.T) {
 	cases := []struct {
 		v        int

model/labels/labels_slicelabels_test.go

@@ -23,3 +23,5 @@ var expectedSizeOfLabels = []uint64{ // Values must line up with testCaseLabels.
 	327,
 	549,
 }
+
+var expectedByteSize = expectedSizeOfLabels // They are identical

model/labels/labels_stringlabels.go

@@ -283,6 +283,13 @@ func (ls Labels) WithoutEmpty() Labels {
 	return ls
 }
 
+// ByteSize returns the approximate size of the labels in bytes.
+// String header size is ignored because it should be amortized to zero
+// because it may be shared across multiple copies of the Labels.
+func (ls Labels) ByteSize() uint64 {
+	return uint64(len(ls.data))
+}
+
 // Equal returns whether the two label sets are equal.
 func Equal(ls, o Labels) bool {
 	return ls.data == o.data

model/labels/labels_stringlabels_test.go

@@ -23,3 +23,12 @@ var expectedSizeOfLabels = []uint64{ // Values must line up with testCaseLabels.
 	270,
 	309,
 }
+
+var expectedByteSize = []uint64{ // Values must line up with testCaseLabels.
+	12,
+	0,
+	37,
+	266,
+	270,
+	309,
+}

model/labels/labels_test.go

@@ -74,6 +74,33 @@ func TestSizeOfLabels(t *testing.T) {
 	}
 }
 
+func TestByteSize(t *testing.T) {
+	require.Len(t, expectedByteSize, len(testCaseLabels))
+	for i, c := range expectedByteSize { // Declared in build-tag-specific files, e.g. labels_slicelabels_test.go.
+		require.Equal(t, c, testCaseLabels[i].ByteSize())
+	}
+}
+
+var GlobalTotal uint64 // Encourage the compiler not to elide the benchmark computation.
+
+func BenchmarkSize(b *testing.B) {
+	lb := New(benchmarkLabels...)
+	b.Run("SizeOfLabels", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			var total uint64
+			lb.Range(func(l Label) {
+				total += SizeOfLabels(l.Name, l.Value, 1)
+			})
+			GlobalTotal = total
+		}
+	})
+	b.Run("ByteSize", func(b *testing.B) {
+		for i := 0; i < b.N; i++ {
+			GlobalTotal = lb.ByteSize()
+		}
+	})
+}
+
 func TestLabels_MatchLabels(t *testing.T) {
 	labels := FromStrings(
 		"__name__", "ALERTS",