[expressions] inspector alerting adapter

[ppisljar]

In order to support alerting on expressions a new adapter should be added to inspector, to which every function producing a render output type should log the received datatable decorated with information about dimensions.

Alerting requires a predictable data structure it can generate alerts on. As every function that outputs render type can output anything we cannot use that directly. One of the options discussed was dropping that last function from expression to get the raw data before it was pre-processed by the chart function, however that has certain limitations for example chart might be receiving the actual data as one of the arguments instead on the input context.

Also, to make it easier for user to configure the alert it would be beneficial if we would know more than just the actual data but dimension information with it, like column 1 is X axis value and column 2 is Y axis value.

All of this information is only available to the chart function that outputs the render context type, so this function should log this data to this new inspector adapter.

Proposed structure on the alerting inspector adapter:

interface AlertingColumn {
  id: string,
  name: string,
  dimension: string,
  field?: string,
}

interface AlertingTable {
  columns: AlertingColumn[]
  rows: DatatableRows,
}
```

[elasticmachine]

Pinging @elastic/kibana-app-arch (Team:AppArch)

[ppisljar]

Possibly we don't need to introduce a new adapter as the datatable adapter behaves pretty much the way we want it. Currently its responsibility of every chart function to log datatable to inspector using handlers.inspectorAdapters.logDatatable. The only missing information at this point is the dimension information, which could be added to the same datatable.

1. we could just use column.name. If column names would be semantic that could give us the information we need. For example imagine xy chart with two metrics, which has a table with 3 columns, the first's name is x-axis, the seconds y-axis Sum of Bytes and the third y-axis Count

chart function could be responsible for providing correct names to the columns before the datatable is sent to inspector.

2. introduce a new meta property on the columns, something like column.meta.dimensionName

[flash1293]

I would prefer the column.meta.dimensionName approach because we can enforce allowed values via types. Using the names feels ad-hoc and we would have to make sure to not modify the table that's passed along because Lens shows column names to the user.