Fix muzzle validation for log4j-2.0 ThreadContextInstrumentation

Author: sarahchen6

dd-java-agent/instrumentation/log4j/log4j-2.0/build.gradle

@@ -26,6 +26,13 @@ muzzle {
     versions = '[,3.0.0)'
   }
 
+  pass {
+    name = 'thread-context'
+    group = 'org.apache.logging.log4j'
+    module = 'log4j-core'
+    versions = '[,2.7)'
+  }
+
   pass {
     group = 'org.apache.logging.log4j'
     module = 'log4j-api'

dd-java-agent/instrumentation/log4j/log4j-2.0/src/main/java/datadog/trace/instrumentation/log4j2/ThreadContextInstrumentation.java

@@ -45,6 +45,11 @@ public String[] helperClassNames() {
     };
   }
 
+  @Override
+  public String muzzleDirective() {
+    return "thread-context";
+  }
+
   public static class ThreadContextAdvice {
     @Advice.OnMethodExit(suppress = Throwable.class)
     public static void mdcClassInitialized() {