[RUM Dashboard] Visitor breakdown charts tooltip

[shahzad31]

We need to add few details like unit to add into tooltip for visitor breakdown tooltip

Right now it's blocked by elastic chart enhancement it doesn't support custom tooltip for partition chart at the moment elastic/elastic-charts#809

[elasticmachine]

Pinging @elastic/kibana-app (Team:KibanaApp)

[elasticmachine]

Pinging @elastic/datavis (Feature:ElasticCharts)

[markov00]

@shahzad31 as described by @nickofthyme here elastic/elastic-charts#809 (comment) you can use a custom tooltip for that