Merge descriptor type and data points into Data. (#202)

This PR does not change any semantics, it just re-organize the protos:
* Removes MetricDescriptor and adds fields to the Metric directly
* Combines MetricDescriptor.Type with specific DataPoints.

This has the following advantages:
* Require only one allocation more than the initial version (v0.4.0). When using gogo proto.
* In-memory representation is smaller than the initial version (v0.4.0).
* Extensible: allows to add new data types in the Metric.
* Groups together metadata and points about different aggregation types.

Signed-off-by: Bogdan Drutu <bogdandrutu@gmail.com>