Add private constructors for stateless marshalers

exporters/otlp/common/src/main/java/io/opentelemetry/exporter/internal/otlp/BoolAnyValueStatelessMarshaler.java

@@ -16,6 +16,8 @@
 final class BoolAnyValueStatelessMarshaler implements StatelessMarshaler<Boolean> {
   static final BoolAnyValueStatelessMarshaler INSTANCE = new BoolAnyValueStatelessMarshaler();
 
+  private BoolAnyValueStatelessMarshaler() {}
+
   @Override
   public void writeTo(Serializer output, Boolean value, MarshalerContext context)
       throws IOException {

exporters/otlp/common/src/main/java/io/opentelemetry/exporter/internal/otlp/DoubleAnyValueStatelessMarshaler.java

@@ -16,6 +16,8 @@
 final class DoubleAnyValueStatelessMarshaler implements StatelessMarshaler<Double> {
   static final DoubleAnyValueStatelessMarshaler INSTANCE = new DoubleAnyValueStatelessMarshaler();
 
+  private DoubleAnyValueStatelessMarshaler() {}
+
   @Override
   public void writeTo(Serializer output, Double value, MarshalerContext context)
       throws IOException {

exporters/otlp/common/src/main/java/io/opentelemetry/exporter/internal/otlp/IntAnyValueStatelessMarshaler.java

@@ -16,6 +16,8 @@
 final class IntAnyValueStatelessMarshaler implements StatelessMarshaler<Long> {
   static final IntAnyValueStatelessMarshaler INSTANCE = new IntAnyValueStatelessMarshaler();
 
+  private IntAnyValueStatelessMarshaler() {}
+
   @Override
   public void writeTo(Serializer output, Long value, MarshalerContext context) throws IOException {
     output.writeInt64(AnyValue.INT_VALUE, value);

exporters/otlp/common/src/main/java/io/opentelemetry/exporter/internal/otlp/StringAnyValueStatelessMarshaler.java

@@ -21,6 +21,8 @@
 final class StringAnyValueStatelessMarshaler implements StatelessMarshaler<String> {
   static final StringAnyValueStatelessMarshaler INSTANCE = new StringAnyValueStatelessMarshaler();
 
+  private StringAnyValueStatelessMarshaler() {}
+
   @Override
   public void writeTo(Serializer output, String value, MarshalerContext context)
       throws IOException {

exporters/otlp/common/src/main/java/io/opentelemetry/exporter/internal/otlp/traces/InstrumentationScopeSpansStatelessMarshaler.java

@@ -23,6 +23,8 @@ final class InstrumentationScopeSpansStatelessMarshaler
   static final InstrumentationScopeSpansStatelessMarshaler INSTANCE =
       new InstrumentationScopeSpansStatelessMarshaler();
 
+  private InstrumentationScopeSpansStatelessMarshaler() {}
+
   @Override
   public void writeTo(
       Serializer output,

exporters/otlp/common/src/main/java/io/opentelemetry/exporter/internal/otlp/traces/ResourceSpansStatelessMarshaler.java

@@ -31,6 +31,8 @@ public final class ResourceSpansStatelessMarshaler
   private static final MarshalerContext.Key SCOPE_SPAN_WRITER_KEY = MarshalerContext.key();
   private static final MarshalerContext.Key SCOPE_SPAN_SIZE_CALCULATOR_KEY = MarshalerContext.key();
 
+  private ResourceSpansStatelessMarshaler() {}
+
   @Override
   public void writeTo(
       Serializer output,

exporters/otlp/common/src/main/java/io/opentelemetry/exporter/internal/otlp/traces/SpanEventStatelessMarshaler.java

@@ -19,6 +19,8 @@
 final class SpanEventStatelessMarshaler implements StatelessMarshaler<EventData> {
   static final SpanEventStatelessMarshaler INSTANCE = new SpanEventStatelessMarshaler();
 
+  private SpanEventStatelessMarshaler() {}
+
   @Override
   public void writeTo(Serializer output, EventData event, MarshalerContext context)
       throws IOException {

exporters/otlp/common/src/main/java/io/opentelemetry/exporter/internal/otlp/traces/SpanLinkStatelessMarshaler.java

@@ -21,6 +21,8 @@
 final class SpanLinkStatelessMarshaler implements StatelessMarshaler<LinkData> {
   static final SpanLinkStatelessMarshaler INSTANCE = new SpanLinkStatelessMarshaler();
 
+  private SpanLinkStatelessMarshaler() {}
+
   @Override
   public void writeTo(Serializer output, LinkData link, MarshalerContext context)
       throws IOException {

exporters/otlp/common/src/main/java/io/opentelemetry/exporter/internal/otlp/traces/SpanStatelessMarshaler.java

@@ -22,6 +22,8 @@
 final class SpanStatelessMarshaler implements StatelessMarshaler<SpanData> {
   static final SpanStatelessMarshaler INSTANCE = new SpanStatelessMarshaler();
 
+  private SpanStatelessMarshaler() {}
+
   @Override
   public void writeTo(Serializer output, SpanData span, MarshalerContext context)
       throws IOException {

exporters/otlp/common/src/main/java/io/opentelemetry/exporter/internal/otlp/traces/SpanStatusStatelessMarshaler.java

@@ -21,6 +21,8 @@
 final class SpanStatusStatelessMarshaler implements StatelessMarshaler<StatusData> {
   static final SpanStatusStatelessMarshaler INSTANCE = new SpanStatusStatelessMarshaler();
 
+  private SpanStatusStatelessMarshaler() {}
+
   @Override
   public void writeTo(Serializer output, StatusData status, MarshalerContext context)
       throws IOException {