proposal contrib/cloud.google.com/go/pubsub.v1: add StartSpanOption to pubsub.Publish

[afarbos]

When sending a message with pubsub we cannot any additional start span options. This block the use of some functions, for example Tag.Measured().

Example:

pr = Publish(ctx , t, msg, Tag.Measured())

Notes: This is one tag that would not be here with default behaviors and block the measure of error rates for examples.