Add support for slices to non-pointers, use non-nullable AnyValue

Before:
```
goos: darwin
goarch: amd64
pkg: go.opentelemetry.io/collector/consumer/pdata
BenchmarkTracesFromOtlp
BenchmarkTracesFromOtlp-16    	     584	   1805875 ns/op	  940352 B/op	   29954 allocs/op
PASS
```

After:
```
goos: darwin
goarch: amd64
pkg: go.opentelemetry.io/collector/consumer/pdata
BenchmarkTracesFromOtlp
BenchmarkTracesFromOtlp-16    	     681	   1575996 ns/op	  857915 B/op	   24802 allocs/op
PASS
```

Signed-off-by: Bogdan Drutu <[email protected]>

Author: bogdandrutu

cmd/pdatagen/internal/base_fields.go

@@ -123,13 +123,13 @@ type baseField interface {
 	generateCopyToValue(sb *strings.Builder)
 }
 
-type sliceField struct {
+type slicePtrField struct {
 	fieldName       string
 	originFieldName string
-	returnSlice     *sliceStruct
+	returnSlice     *slicePtrStruct
 }
 
-func (sf *sliceField) generateAccessors(ms baseStruct, sb *strings.Builder) {
+func (sf *slicePtrField) generateAccessors(ms baseStruct, sb *strings.Builder) {
 	sb.WriteString(os.Expand(accessorSliceTemplate, func(name string) string {
 		switch name {
 		case "structName":
@@ -146,7 +146,7 @@ func (sf *sliceField) generateAccessors(ms baseStruct, sb *strings.Builder) {
 	}))
 }
 
-func (sf *sliceField) generateAccessorsTest(ms baseStruct, sb *strings.Builder) {
+func (sf *slicePtrField) generateAccessorsTest(ms baseStruct, sb *strings.Builder) {
 	sb.WriteString(os.Expand(accessorsSliceTestTemplate, func(name string) string {
 		switch name {
 		case "structName":
@@ -161,15 +161,63 @@ func (sf *sliceField) generateAccessorsTest(ms baseStruct, sb *strings.Builder)
 	}))
 }
 
-func (sf *sliceField) generateSetWithTestValue(sb *strings.Builder) {
+func (sf *slicePtrField) generateSetWithTestValue(sb *strings.Builder) {
 	sb.WriteString("\tfillTest" + sf.returnSlice.structName + "(tv." + sf.fieldName + "())")
 }
 
-func (sf *sliceField) generateCopyToValue(sb *strings.Builder) {
+func (sf *slicePtrField) generateCopyToValue(sb *strings.Builder) {
 	sb.WriteString("\tms." + sf.fieldName + "().CopyTo(dest." + sf.fieldName + "())")
 }
 
-var _ baseField = (*sliceField)(nil)
+var _ baseField = (*slicePtrField)(nil)
+
+type sliceValueField struct {
+	fieldName       string
+	originFieldName string
+	returnSlice     *slicePtrStruct
+}
+
+func (sf *sliceValueField) generateAccessors(ms baseStruct, sb *strings.Builder) {
+	sb.WriteString(os.Expand(accessorSliceTemplate, func(name string) string {
+		switch name {
+		case "structName":
+			return ms.getName()
+		case "fieldName":
+			return sf.fieldName
+		case "returnType":
+			return sf.returnSlice.structName
+		case "originFieldName":
+			return sf.originFieldName
+		default:
+			panic(name)
+		}
+	}))
+}
+
+func (sf *sliceValueField) generateAccessorsTest(ms baseStruct, sb *strings.Builder) {
+	sb.WriteString(os.Expand(accessorsSliceTestTemplate, func(name string) string {
+		switch name {
+		case "structName":
+			return ms.getName()
+		case "fieldName":
+			return sf.fieldName
+		case "returnType":
+			return sf.returnSlice.structName
+		default:
+			panic(name)
+		}
+	}))
+}
+
+func (sf *sliceValueField) generateSetWithTestValue(sb *strings.Builder) {
+	sb.WriteString("\tfillTest" + sf.returnSlice.structName + "(tv." + sf.fieldName + "())")
+}
+
+func (sf *sliceValueField) generateCopyToValue(sb *strings.Builder) {
+	sb.WriteString("\tms." + sf.fieldName + "().CopyTo(dest." + sf.fieldName + "())")
+}
+
+var _ baseField = (*sliceValueField)(nil)
 
 type messagePtrField struct {
 	fieldName       string