Review fixes for restlet-2.0:library (#17015)

Co-authored-by: otelbot <197425009+otelbot@users.noreply.github.com>

Author: otelbot[bot]

instrumentation/restlet/restlet-2.0/library/src/main/java/io/opentelemetry/instrumentation/restlet/v2_0/TracingFilter.java

@@ -21,7 +21,7 @@ final class TracingFilter extends Filter {
   private final Instrumenter<Request, Response> instrumenter;
   private final String path;
 
-  public TracingFilter(Instrumenter<Request, Response> instrumenter, String path) {
+  TracingFilter(Instrumenter<Request, Response> instrumenter, String path) {
     this.instrumenter = instrumenter;
     this.path = path;
   }

instrumentation/restlet/restlet-2.0/library/src/main/java/io/opentelemetry/instrumentation/restlet/v2_0/internal/RestletHeadersGetter.java

@@ -53,21 +53,22 @@ public Iterable<String> keys(Request carrier) {
   }
 
   @Override
-  public String get(Request carrier, String key) {
+  @Nullable
+  public String get(@Nullable Request carrier, String key) {
     Series<?> headers = getHeaders(carrier);
     return headers == null ? null : headers.getFirstValue(key, /* ignoreCase= */ true);
   }
 
   @Override
-  public Iterator<String> getAll(Request carrier, String key) {
+  public Iterator<String> getAll(@Nullable Request carrier, String key) {
     Series<?> headers = getHeaders(carrier);
     return headers == null
         ? emptyIterator()
         : asList(headers.getValuesArray(key, /* ignoreCase= */ true)).iterator();
   }
 
   @Nullable
-  static Series<?> getHeaders(Message carrier) {
+  static Series<?> getHeaders(@Nullable Message carrier) {
     if (GET_ATTRIBUTES == null) {
       return null;
     }

instrumentation/restlet/restlet-2.0/library/src/main/java/io/opentelemetry/instrumentation/restlet/v2_0/internal/RestletHttpAttributesGetter.java

@@ -16,8 +16,7 @@
 import org.restlet.Response;
 import org.restlet.util.Series;
 
-enum RestletHttpAttributesGetter implements HttpServerAttributesGetter<Request, Response> {
-  INSTANCE;
+final class RestletHttpAttributesGetter implements HttpServerAttributesGetter<Request, Response> {
 
   @Override
   public String getHttpRequestMethod(Request request) {

instrumentation/restlet/restlet-2.0/library/src/main/java/io/opentelemetry/instrumentation/restlet/v2_0/internal/RestletTelemetryBuilderFactory.java

@@ -22,7 +22,7 @@ public static DefaultHttpServerInstrumenterBuilder<Request, Response> create(
     return DefaultHttpServerInstrumenterBuilder.create(
         "io.opentelemetry.restlet-2.0",
         openTelemetry,
-        RestletHttpAttributesGetter.INSTANCE,
+        new RestletHttpAttributesGetter(),
         new RestletHeadersGetter());
   }
 }