RFR: Add support for jdk.jfr.Frequency

[Jie Kang]

Hi,

The patch attached adds support for the annotation jdk.jfr.Frequency,
using the Hertz unit introduced in JMC-5768 for JFR events like
CPUTimeStampCounter, ThreadContextSwitchRate, etc.

The following is a before and after image for the event browser for
CPUTimeStampCounter.
https://imgur.com/a/cSVlmOM

How does it look?

I was initially also trying to find related unit tests, but I don't
think any exist. I will be looking to write some but I'd like some
feedback if this is even a correct implementation before continuing.


Regards,
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