Implement APIs and Data Model

[jparsana]

APIs / Data Model across languages should match

There are two main components of the Stats APIs - tags and stats. The stats component provides the framework to define measures (e.g. rpc client latency) and record stats against those measures. The tags component provides a way to break down the data for a measure along user-defined dimensions. Finally, views, which are part of stats, all users to specify the aggregations of the stats that they are interested in. The language-independent specifications of tags and stats are available on opencensus-specs, specifically:

OpenCensus Tags
OpenCensus Stats

[basvanbeek]

First version is awaiting review / discussion in #220

[basvanbeek]

#220 has been merged