InferredProxySpanGroup should be static class to avoid unnecessary dependency on surrounding decorator

Author: mcculls

dd-java-agent/agent-bootstrap/src/main/java/datadog/trace/bootstrap/instrumentation/decorator/HttpServerDecorator.java

@@ -56,7 +56,7 @@
 public abstract class HttpServerDecorator<REQUEST, CONNECTION, RESPONSE, REQUEST_CARRIER>
     extends ServerDecorator {
 
-  class InferredProxySpanGroup implements AgentSpan {
+  static class InferredProxySpanGroup implements AgentSpan {
     private final AgentSpan inferredProxySpan;
     private final AgentSpan serverSpan;