I am trying to create in my dashboard widget with type table, where I can show more than one value for specific operation. I am going through MQL documentation but I failed to find anything useful
Right now I tried to use join or union but the best I got is this
{ fetch k8s_container
| metric 'custom.googleapis.com/my_metric/count'
| filter (metric.error == 'error')
| group_by 1d
| every 1m
| group_by [metric.operation, metric.error];
fetch k8s_container
| metric 'custom.googleapis.com/my_metric/count'
| group_by 1d
| every 1m
| group_by [metric.operation, metric.error], [row_count: row_count()]
} | outer_join 0, 0
In other tries, values has been added to each other or multiplied.
My goal is to show data similar to this table below

Is it possible to add few values on one table? I would be grateful for any tip or any link to valuable resource/example/documentation

You can create a custom table representation using time series widgets and add multiple time series for specific operations in your custom dashboard.
For adding charts and tables to a custom dashboard, see this document.