all: refactor by extraction code for conversion from timestampStr to microseconds

I noticed while reviewing https://github.com/census-instrumentation/opencensus-python/pull/327#discussion_r220030262 that there is a legacy duplication of code to convert between timestamp strings to microseconds that is https://github.com/census-instrumentation/opencensus-python/blob/c96083589792a0c278193af1a227c21f9c6c6e97/opencensus/trace/exporters/zipkin_exporter.py#L154-L167

and with that PR there will be three sites with the same code pattern.

I propose that we refactor that code by extracting it to a small helper function

def datetimeStrToMicroseconds(timestamp):
   """
   Convert a timestamp string into a microsecond value as required by Zipkin.
   It returns an int
   """
   dt = datetime.datetime.strptime(timestamp, ISO_DATETIME_REGEX)
   mUs = (calendar.timegm(dt.timetuple()) * 1e6) + dt.microsecond
   return int(round(mUs))

which will then reduce all those call sites to simply

start_timestamp_mus = datetimeStrToMicroseconds(span.start_time)
end_timestamp_mus = datetimeStrToMicroseconds(span.end_time)
epoch_time_mus = datetimeStrToMicroseconds(time_event.timestamp)

[odeke-em]

Great catch, perhaps then we need to put that in the utilities path if any exists, so that we can use that function across the entire repo.

[mayurkale22]

We should do same here exporter/jaeger: https://github.com/census-instrumentation/opencensus-python/blob/c96083589792a0c278193af1a227c21f9c6c6e97/opencensus/trace/exporters/jaeger_exporter.py#L179-L189