Stats: Add an API to record measurements against implicit context.

[songy23]

See OpenCensus-Specs:

Users should record Measurements against a context, either an explicit context or the implicit current context.

Currently there's only an API that records measurements against explicit context: https://github.com/census-instrumentation/opencensus-python/blob/master/opencensus/stats/measurement_map.py#L50-L57.