RFR: 8037914: Add JFR event for string deduplication

[Francesco Andreuzzi]

On Thu, 30 Oct 2025 06:01:48 GMT, Erik Gahlin <egahlin at openjdk.org> wrote:

> It would be good if you could provide some ballpark figures on the number of events in a worst-case scenario, so we can determine what GC level is appropriate.

I wrote a simple pathological test with multiple threads interning random strings, [this](https://github.com/user-attachments/files/23282465/out-parallel.txt) is the worst I've seen: 100 deduplication rounds within `3.698s` and `3.729s`.

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PR Comment: https://git.openjdk.org/jdk/pull/28015#issuecomment-3476289368