Terraform for Firestore creation

See https://cloud.google.com/firestore/docs/solutions/automate-database-create#create_a_database_with_terraform

Set the database_type to CLOUD_FIRESTORE or CLOUD_DATASTORE_COMPATIBILITY.

provider "google" {
  credentials = file("credentials-file")
}

resource "google_project" "my_project" {
  name = "My Project"
  project_id = "project-id"
}

resource "google_app_engine_application" "app" {
  project     = google_project.my_project.project_id
  location_id = "location"
  database_type = "CLOUD_FIRESTORE"
}

Update 7/23/20: see automating database creation.

You can enable Firestore using the google_project_service resource:

resource "google_project_service" "firestore" {
  project = var.project_id
  service = "firestore.googleapis.com"

  disable_dependent_services = true
}

Edit: I don't see any possibility to create the database itself, however, you can use the google_firebase_project_location to set the location of the Firestore (this will also set the GAE location and the location of the default bucket).