[TSDB] Investigate how we can simplify the TSDB mapping

[gmarouli]

Description

Currently, a user needs to specify the dimensions and the metrics in the mapping of a time series index/data stream. Furthermore, the user can also have fields that are neither.

To define a dimension the user needs to set "time_series_dimension": true, to specify a metric the user needs to set the type of metric: "time_series_metric": "counter" or "time_series_metric": "gauge".

We want to investigate if we can reduce this user burden and determine as much as we can dynamically.

[elasticsearchmachine]

Pinging @elastic/es-storage-engine (Team:StorageEngine)