Mismatch in event publish_time

below is the example code, where we are trying to get the publish_time of the pubsub message in our DoFn(). It seems the type has changed in the version of apache beam starting 2.36.0. However, I was not able to find any release notes that talk about this change. Any reference will be helpful.

```

class ProtoToDictDoFn(beam.DoFn):
def process(self, element, publish_time=beam.DoFn.TimestampParam):

"""
element is type PubsubMessage(),
"""
print('-------------')
print(type(element.publish_time))

print(element.publish_time)
print('-------------')

Output:
— for version 2.35.0 —

seconds: 1652814206
nanos: 417000000

— for version >=
2.36.0 —

2022-05-17 19:02:06.314000+00:00

```

Which seems to be an issue. As per the [google pubsub documentation publish time should be type "google.protobuf.timestamp_pb2.Timestamp"](https://github.com/googleapis/python-pubsub/blob/main/google/pubsub_v1/types/pubsub.py#L232)

Any clue, what has changed or caused this issue?

Imported from Jira [BEAM-14482](https://issues.apache.org/jira/browse/BEAM-14482). Original Jira may contain additional context.
Reported by: daljeet.