JFR is leaking strings

Every time JFR is executed a string with a review of the operation remains in
the heap.

This is not relevant when JFR is executed few times, but this customer calls
permanently to JFR from WLS every 5 minutes; this is 288 times a day. After
some days, the list of strings associated to JRF is relevant producing
eventually an OOM.

This issue is independent of the Java app, we have tested with WLS 10.3.6,
WLS 12.2.1.2 and a simple Java app. It happens on JDK7 and JDK8.
Assistance of JDK team is required to fix this issue.