Right now, if a metric is missing in Elasticsearch, we return an error: custom metric <xxx> is not served by any metric client.
This is causing issues in environments with little traffic, where the metric could be missing just because there hasn't been any requests to generate it.

By making it an error, we prevent HPAs from fully synchronizing with no errors.
We should instead return 0, to avoid that error.
We may also want to log a debug/warning when this happens, just to ensure visibility.

cc @barkbay