Fix panic in StackDriver Trace Exporter (#332)
Fix panic in StackDriver Trace Exporter (#332)

Signed-off-by: Andrea Luzzardi <442ffe3ffbfd024afcf0b7f8b108d05bca3aae43@gmail.com>
census-instrumentation/opencensus-go,census-instrumentation/opencensus-go,census-instrumentation/opencensus-go