Drop negative bucket bounds and negative values in record API

[mayurkale22]

Negative values are currently not supported by any of the backends that we support and have histograms (Stackdriver and Prometheus). We have decided to implement a backwards incompatible change and drop negative values in the Record API (We will start with logging an error for few versions then throw an exception).

This could be a breaking change if users are recording negative value for any measure. We planned to implement this change asap in all the libraries and explain this behaviour to the current users very clearly.

Counterpart in Java: census-instrumentation/opencensus-java#1499 and census-instrumentation/opencensus-java#1525