Rewrite stats exporters to use Metrics instead of Stats (ViewData) #257

The plan is to use Metric data model to transform data from Stats API (Stats data models to Metric data models handled by MetricUtils class) and Gauge APIs using Metric Producer Manager.

• Rewrite/update Stackdriver stats exporter.
• Rewrite/update Prometheus stats exporter.

Way to translate Stats data model to the Metrics.

• MetricPoint
  • MetricDescriptor
    • Name <- ViewData.View.name
    • Description <- ViewData.View.description
    • Unit <- ViewData.View.Measure.unit
    • Type <- Cumulative (except LastValue which is GAUGE)
  • TimeSeries
    • Labels <- {ViewData.rows, ViewData.View.columns}
• CumulativePoint
  • Value <- ViewData.AggregationData.value (for DistributionValue the bucket_bounds are defined in the ViewData.View.DistributionAggregation.bucket_bounds)
  • Start_time <- ViewData.start_time
  • End_time <- ViewData.end_time

Follow-up of #253

Equivalent change in Java: census-instrumentation/opencensus-java#1501.