RFR: 8185525: [Event Request] Add Tracing event for DictionarySizes

[Erik Gahlin]

Looks good.

Since the tracing framework has been replaced with JFR and there is a 
JIRA tag for "jfr-event-request", we could perhaps remove "[Event 
Request]" and replace "tracing" with "JFR" in the bug title.

Thanks
Erik

> hi all,
>
> Please review this, hopefully final, fix. After some offline 
> discussion, we agreed to:
>
> #1 Keep the rate attributes (can't synthesize otherwise because we 
> don't track additions/deletions on their own)
>
> #2 Use data from last event if we can't lock a table (fulfills JFR 
> contract in which we need to deliver event at specified period)
>
> Bug:     https://bugs.openjdk.java.net/browse/JDK-8185525
> Webrev:  http://cr.openjdk.java.net/~gziemski/8185525_rev9
> Testing: Mach5 tier1,2,3 (another Mach5 tier1,2,3,4,5,6,7 in progress…)
>
> cheers