metrics server failure missed by tests

We do test metrics in some capacity now:

@pytest.mark.asyncio
@pytest.mark.flaky(max_runs=5, min_passes=1)
async def test_toggle_metrics(model, tools):
    """Turn metrics on/off via the 'enable-metrics' config on kubernetes-master,
    and check that workload status returns to 'active', and that the metrics-server
    svc is started and stopped appropriately.

    """

    async def check_svc(app, enabled):
        unit = app.units[0]
        if enabled:
            await retry_async_with_timeout(
                verify_ready,
                (unit, "svc", ["metrics-server"], "-n kube-system"),
                timeout_msg="Unable to find metrics-server svc before timeout",
            )
        else:
            await retry_async_with_timeout(
                verify_deleted,
                (unit, "svc", "metrics-server", "-n kube-system"),
                timeout_msg="metrics-server svc still exists after timeout",
            )

    app = model.applications["kubernetes-master"]

    k8s_version_str = app.data["workload-version"]
    k8s_minor_version = tuple(int(i) for i in k8s_version_str.split(".")[:2])
    if k8s_minor_version < (1, 16):
        click.echo("skipping, k8s version v" + k8s_version_str)
        return

    config = await app.get_config()
    old_value = config["enable-metrics"]["value"]
    new_value = not old_value

    await set_config_and_wait(
        app, {"enable-metrics": str(new_value)}, tools, timeout_secs=240
    )
    await check_svc(app, new_value)

    await set_config_and_wait(
        app, {"enable-metrics": str(old_value)}, tools, timeout_secs=240
    )
    await check_svc(app, old_value)

If this is enough we can close this bug

That test checks to see if the metrics-server Service gets created, but it doesn't really test to make sure metrics are working. It wouldn't cover the case that was originally reported.