InfluxDBBackendListener: Ability to send user tags based on running test

@pmouawad ([Bug 65927](https://bz.apache.org/bugzilla//show_bug.cgi?id=65927&redirect=false)):
Currently in InfluxDBBackendListenerClient, user can add tags to the data sent using TAG_ prefixed key.

This userTag attribute value is computed on start of test (https://github.com/apache/jmeter/blob/master/src/components/src/main/java/org/apache/jmeter/visualizers/backend/influxdb/InfluxdbBackendListenerClient.java#L361) which limits a bit what users can add to it.

For example, a user reported to me that using ${__threadGroupName} was not working, which is in fact not supported since userTag is computed on test start and besides InfluxDBBackendListenerClient is called in another thread than User Thread Groups.

See for example how this limitation has been workaround:

- https://github.com/delirius325/jmeter-elasticsearch-backend-listener/wiki/Sending-JMeter-variables

OS: All