Add "Positive Rate" as a metric aggregation

[Pigueiras]

Is your feature request related to a problem? Please describe.
I would like to see rates calculated over cumulative metrics in Grafana using OpenSearch as a backend.

Describe the solution you'd like
With the TSVB panel in OpenSearch Dashboards, you can use more metrics and pipeline aggregations than are currently available in Grafana. One such aggregation is called "Positive Rate" to calculate rates is a feature that I find missing in OpenSearch grafana data source and would like to see implemented.

[sarahzinger]

Hi @Pigueiras thanks for the suggestion, sounds like a good one!

Putting this in the backlog for now until we're ready to look at new feature requests.