Add default value to metric.filter in the resource google_compute_autoscaler

[modular-magician]

Fixes hashicorp/terraform-provider-google#15033

Add default value to metric.filter in the resource google_compute_autoscaler

API: https://cloud.google.com/compute/docs/reference/rest/v1/autoscalers

The API doc mentions:
If not specified, the type defaults to gce_instance.

If this PR is for Terraform, I acknowledge that I have:

• Searched through the issue tracker for an open issue that this either resolves or contributes to, commented on it to claim it, and written "fixes {url}" or "part of {url}" in this PR description. If there were no relevant open issues, I opened one and commented that I would like to work on it (not necessary for very small changes).
• Ensured that all new fields I added that can be set by a user appear in at least one example (for generated resources) or third_party test (for handwritten resources or update tests).
• Generated Terraform providers, and ran make test and make lint in the generated providers to ensure it passes unit and linter tests.
• Ran relevant acceptance tests using my own Google Cloud project and credentials (If the acceptance tests do not yet pass or you are unable to run them, please let your reviewer know).
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Release Note Template for Downstream PRs (will be copied)

compute: added default value to `metric.filter` in the resource `google_compute_autoscaler` (beta)

Derived from GoogleCloudPlatform/magic-modules#8327