BackendListener: Allow using runtime variables

**sergiy.iampol** ([Bug 63478](https://bz.apache.org/bugzilla//show_bug.cgi?id=63478&redirect=false)):
trying to use BackendListener and observed runtime variables are not written to influxDB.

Properties, on the other hand, can be written to influx.

So I can separate test results by some ID by setting measurement=${__P(SOME_ID)}

What I'm looking for is a splitting results by thread group name, as I may have up to several dozens of them within the same test.

Tried to use following:
TAG_scenarioName=${__threadGroupName}
TAG_someJmeterVar=${SOME_JMETER_VAR}
TAG_someJmeterVarAsGroovy=${__groovy(vars.get("SOME_JMETER_VAR"),)}
eventTags=${__threadGroupName}
testTitle=${__threadGroupName} (this one makes less sense, but still..)

and none of those works

Those are works:
TAG_injectorName=${__machineName()}
TAG_predefinedVar=${USER_DEFINED_VAR} (I believe this is thanks to https://github.com/apache/jmeter/issues/3600)

So as I understand problem is with runtime variables only. Is it possible to make runtime variables accessible for the BackendListener? Or maybe there is some workaround for such case?

OS: All