file_exporter can't append

[cjd9023]

from opencensus.trace.exporters import file_exporter
from opencensus.trace.tracer import context_tracer (not tracers)

exporter = file_exporter.FileExporter(file_name='/tmp/traces')
tracer = context_tracer.ContextTracer(exporter=exporter)

lib/python2.7/site-packages/opencensus/trace/exporters/file_exporter.py

   with open(self.file_name, 'w+') as file:
        trace_str = json.dumps(trace)
        file.write(trace_str)

i change to :

    with open(self.file_name, 'a+') as file:
        trace_str = json.dumps(trace)
        file.write(trace_str)
        file.write('\n')