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

[mayurkale22]

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.

[mayurkale22]

Rewrite/update Prometheus stats exporter. - On Hold

Currently, prom-client for node is not compatible with OpenCensus Metrics data model. Basically, prom-client doesn't seem to support custom collectors. I have opened an issue: siimon/prom-client#241 to track this.