Extend Beta for Metric Preaggregation APIs

[macrogreg]

We shipped a lot of new Metric APIs in 2.6.0-beta3.
(Here are some docs: #774, #784)

This is a big change and we encourage everybody to take a look at the new APIs and to try them out.
In order to give ourselves more leeway in reacting to incoming Beta-feedback we will hold off on shipping these APIs as a part of the upcoming 2.6 stable release. Instead, we will include these APIs into the 2.7-Beta1 release that will be shipped at the same time. Subsequently, the new metric aggregation APIs will be included into the 2.7 stable release.

More information about the upcoming SDK release schedule can be found here:
https://github.com/Microsoft/ApplicationInsights-Home/wiki/SDK-Release-Schedule