Add Metrics APIs as per "metrics.proto"

[mayurkale22]

Add Metrics package and below classes based on metrics.proto. Although Metrics proto is still being finalized, We think it is safe to start working on value classes like LabelKey, TimeSeries, Point etc.

• Metrics: Add LabelKey and LabelValue.
• Metrics: Add Value and Point.
• Metrics: Add TimeSeries.
• Metrics: Add MetricDescriptor.
• Metrics: Add Metric.
• Metrics: Add Summary and Distribution.
• Metrics: Produce and store Metrics in Stats impl.

Counterpart classes available in Java for reference : https://github.com/census-instrumentation/opencensus-java/tree/master/api/src/main/java/io/opencensus/metrics