feat: allow set filter on mig autoscaling metric

[davidmontoyago]

for modules/mig, allow setting filter on the autoscaling metric in order to scale a group with a custom metric selector.

this would be useful for module terraform-google-modules/terraform-google-github-actions-runners to allow scaling groups of Github runners with custom/external metrics.

[davidmontoyago]

Hi @ayushmjain @erlanderlo @q2w @apeabody 👋 , can I get a review? this would ultimately allow an invoker to set a metric with a custom filter like this:

autoscaling_metric = [{
  name   = google_monitoring_metric_descriptor.my_github_actions_custom_metric.type,
  target = 1,
  type   = "GAUGE",
  filter = "resource.type = \"global\" AND resource.labels.project_id = \"my-custom-metric-project\" AND metric.labels.repository = \"my-metric-repo\""
}]

note that this was attempted before via #363 but the provider wasn't ready.