Kafka update related to JVM metrics and legend fix

[v-zhuravlev]

• Update and explain metricsSourceoptions related to kafka JVM metrics
• Fix BytesInPerSec metrics for prometheus
• Fix topic dashboard(was not showing any consumergroups due to nonexistent consumergroup variable)
• Improve aggLegend: show only single label
  • Update zookeeper legends
  • Update legends in kafka-observ-lib
• Move memoryUsedVirtual to sep panel