Performance: Formatter is called for each data point in bar chart even if not used

[flash1293]

Describe the issue
In https://github.com/elastic/elastic-charts/blob/master/packages/charts/src/chart_types/xy_chart/rendering/bars.ts#L92 the provided formatter is called for each data point in the series, but it's only used in special circumstances:

https://github.com/elastic/elastic-charts/blob/master/packages/charts/src/chart_types/xy_chart/rendering/bars.ts#L96

In case of charts with lots of series and slow formatting functions provided this can have a noticable performance impact.

E.g. in the following chart it's spending 44ms in getConvertedValue (out of ~360ms total blocking rendering time):

To Reproduce
Configure a bar chart without labels on the bars and observe the provided formatter being called.

Expected behaviour
Formatter should only be called if necessary to render the chart correctly

[markov00]

Thanks Joe, yep, this can definitely be improved a lot
We can probably also call the value formatting only when showing the tooltip, without having a precomputed value saved