Overwrite data for specific metrics using API

[#1]

It would be great to be able to send data via the API (1.4) to only update a specific metric associated with a particular dimension value.

We use data sources to push data into Analytics; for a particular dimension we have multiple values, each containing multiple metrics.

We've run into issues with one specific metric for multiple dimension values and we don't have a method to update that specific metric without sending the entire data set unless we set up a new data source.