[Lens] Field Formatters not working for multiple layers

[LucaWintergerst]

Kibana version: 7.7.0

Elasticsearch version: 7.7.0

Server OS version: Elastic Cloud

Browser version: Chrome 83

Browser OS version: MacOs

Original install method (e.g. download page, yum, from source, etc.): Elastic Cloud

Describe the bug: Field Formatters only work for a single metric in Kibana, not for multiple layers

Steps to reproduce:

1. visualize a field with a bytes field formatter => looks good
2. add second layer with a field, also using a field formatter of type bytes => no longer displaying Bytes
   Expected behavior:

Screenshots (if relevant):

Errors in browser console (if relevant):

Provide logs and/or server output (if relevant):

Any additional context:

[elasticmachine]

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

[flash1293]

This is happening on purpose right now because there is just a single axis and if there are multiple field formatters, it's not clear which one to apply:

kibana/x-pack/plugins/lens/public/xy_visualization/xy_expression.tsx

Line 216 in 84ed509

// use default number formatter for y axis and use formatting hint if there is just a single y column

This behavior is safe (we won't ever display a dollar amount on a byte axis) but it's probably not the best one for cases like the one in the example with multiple metrics in different layers which actually use the same formatter.

The least we can do is to check whether the formatters of the different metric columns are identical. For other cases we should implement #53663 which would just split into two different axes if the formatters are different.