AMS unit ended up to error status when specifying the prometheus_extra_labels config item

[Gary.Wang]

When deploying the ams with the
```
 prometheus_extra_labels: region=subcluster-1
```

config item, we saw the following error from the ams/0 unit
```
File "/var/lib/juju/agents/unit-ams-0/charm/./src/charm.py", line 546, in _on_config_changed
    self.ams.configure(cfg)
  File "/var/lib/juju/agents/unit-ams-0/charm/src/ams.py", line 231, in configure
    rendered_content = template.render(content)
  File "/var/lib/juju/agents/unit-ams-0/charm/venv/jinja2/environment.py", line 1295, in render
    self.environment.handle_exception()
  File "/var/lib/juju/agents/unit-ams-0/charm/venv/jinja2/environment.py", line 942, in handle_exception
    raise rewrite_traceback_stack(source=source)
  File "templates/settings.yaml.j2", line 32, in top-level template code
    {%- for key, value in metrics.extra_labels.items() %}
  File "/var/lib/juju/agents/unit-ams-0/charm/venv/jinja2/utils.py", line 92, in from_obj
    if hasattr(obj, "jinja_pass_arg"):
jinja2.exceptions.UndefinedError: 'str object' has no attribute 'items'
2025-03-29 07:40:05 ERROR juju.worker.uniter.operation runhook.go:180 hook "etcd-relation-changed" (via hook dispatching script: dispatch) failed: exit status 1
```