There were 4 counters added to measure the impact of locking on loading classes. The first is rarely non-zero. Are there tools that use these counters? As SUN_CLS counters, they are internal counters and there's no expectation of support.
From: perfData.hpp

 * sun.* - unstable, unsupported interface

// The following performance counters are added for measuring the impact
// of the bug fix of 6365597. They are mainly focused on finding out
// the behavior of system & user-defined classloader lock, whether
// ClassLoader.loadClass/findClass is being called synchronized or not.
NEWPERFEVENTCOUNTER(_sync_systemLoaderLockContentionRate, SUN_CLS, "systemLoaderLockContentionRate");
NEWPERFEVENTCOUNTER(_sync_nonSystemLoaderLockContentionRate, SUN_CLS, "nonSystemLoaderLockContentionRate");
NEWPERFEVENTCOUNTER(_sync_JVMFindLoadedClassLockFreeCounter, SUN_CLS, "jvmFindLoadedClassNoLockCalls");
NEWPERFEVENTCOUNTER(_sync_JVMDefineClassLockFreeCounter, SUN_CLS, "jvmDefineClassNoLockCalls");
NEWPERFEVENTCOUNTER(_sync_JNIDefineClassLockFreeCounter, SUN_CLS, "jniDefineClassNoLockCalls");