Is your feature request related to a problem? Please describe.

I'm adding metrics to my database watch client. I want to know how many of each type of event we are seeing.

DocumentChangeKind is an integer so if we include that in our metric, we will see a number instead of a readable kind.

Describe the solution you'd like

Add a .String() method to type DocumentChangeKind int so that we can see the name of the event kind instead of a number.

I'd like to make a PR for this, if it is approved. Here's the code I would add:

func (k firestore.DocumentChangeKind() string {
	switch k {
	case firestore.DocumentAdded:
		return "Added"
	case firestore.DocumentModified:
		return "Modified"
	case firestore.DocumentRemoved:
		return "Removed"
	default:
		return "Unknown"
	}
}

Describe alternatives you've considered

Maintaining my own would be annoying and problematic if Firestore adds more event kinds.