Add class-scope "FromNanoseconds", "FromMicroseconds", "FromMilliseconds", "FromSeconds", "FromDatetime", and "FromJsonString" methods

[nathanielmanistaatgoogle]

To what language does this apply? Python

Describe the problem you are trying to solve. Construct a google.protobuf.timestamp_pb2.Timestamp from a numeric value of milliseconds in the most-attractive, with-the-best-developer-experience possible.

Describe the solution you'd like my_timestamp_message = timestamp_pb2.Timestamp.FromMilliseconds(my_numeric_value)

Describe alternatives you've considered my_timestamp_message = timestamp_pb2.timestamp_from_milliseconds(my_numeric_value) would probably be okay.

Protocol Buffers Python currently only supports

my_timestamp_message = timestamp_pb2.Timestamp()
my_timestamp_message.FromMilliseconds(my_numeric_value)

. That there is not a single-expression way to construct a populated-from-numerical-value-of-milliseconds timestamp_pb2.Timestamp message means that lots of opportunities for attractive code are being missed (and additional temporary variables and helper methods written...).

Additional context Since FromMilliseconds is currently an instance-scope method, adding a class-scope method of the same name might require some nicely-encapsulated-and-not-marring-the-public-API hackery like

class Example(object):
    def __init__(self):
        self.FromMilliseconds = self.InstanceScopeFromMilliseconds

    def InstanceScopeFromMilliseconds(self, milliseconds):
        # (some implementation)

    @classmethod
    def FromMilliseconds(cls, milliseconds):
        # (some implementation that returns an instance of the class)

Obviously there's nothing special about FromMilliseconds; this enhancement should also be done for FromNanoseconds, FromMicroseconds, FromSeconds, FromDatetime, and FromJsonString.

Issue 3986 is something like this in that it asks for something that allows timestamp_pb2.Timestamp instantiation on one line, but I don't agree with the one concrete suggestion that it offers (returning self from instance-scope FromMilliseconds; yuck!).