fix ZipkinExporter when using BackgroundThreadTransport (#247)

* changed time precision on zipkin exporter to microseconds

* fixed tests

* fixed line length

* fixed translate_to_zipkin function on ZipkinExporter
- previously when using BackgroundThreadTransport, all traces were sent with same trace_id

Author: felippe-mendonca

opencensus/trace/exporters/zipkin_exporter.py

@@ -137,10 +137,6 @@ def translate_to_zipkin(self, span_datas):
         :returns: List of zipkin format spans.
         """
 
-        top_span = span_datas[0]
-        trace_id = top_span.context.trace_id if top_span.context is not None \
-            else None
-
         local_endpoint = {
             'serviceName': self.service_name,
             'port': self.port,
@@ -173,7 +169,7 @@ def translate_to_zipkin(self, span_datas):
             duration_ms = end_timestamp_ms - start_timestamp_ms
 
             zipkin_span = {
-                'traceId': trace_id,
+                'traceId': span.context.trace_id,
                 'id': str(span.span_id),
                 'name': span.name,
                 'timestamp': int(round(start_timestamp_ms)),