Define Metric SDK behavior when presented with invalid metric recording values

[jkwatson]

1. We know that the metrics API cannot throw exceptions/panic on bad inputs.
2. The Counter (for example) cannot accept negative values to its add operation. Histograms should only accept positive values for recordings.
3. How should SDK implementations behave when presented with syntactically valid but semantically invalid inputs such as this for metric recordings?

[reyang]

Covered by #2007.