How to receive publishTime

It's not very well documented, but it is.

The event structure is different when you have a push to the function directly compare to a push Subscription with a HTTP triggered function.

Instead, you can access publishTime and messageId via the event ID and timestamp properties of the event metadata. This metadata is accessible via the context object that is passed to your function when it is invoked.

In the background context description, in Go, there is an explanation of the package to use (functions/metadata) to get the context values

Here how to achieve this

1. Perform a go get cloud.google.com/go
2. Use this dependency in your code and get the message metadata from the context

import (
    "cloud.google.com/go/functions/metadata"
    "context"
    "log"
)

type PubSubMessage struct {
    Data []byte `json:"data"`
}

// HelloPubSub consumes a Pub/Sub message.
func HelloPubSub(ctx context.Context, m PubSubMessage) error {
    meta,err := metadata.FromContext(ctx)
    if err != nil {
        log.Println(err)
        return err
    }
    log.Printf("publishTime, %s", meta.Timestamp)
    log.Printf("messageId, %s", meta.EventID)
    log.Printf("Type, %s", meta.EventType)
    log.Printf("Resource, %s", meta.Resource)
    return nil
}