Hi @Christian Fabián ,
AFAIK, you could query your custom metrics in Metrics Storage via Azure Workbooks. This is a known limitation of the Preview feature.
When you enable "Monitor Coverage" it means you create a data collection rule for VM Insights solution, it collects standard list of performance counters into InsightsMetrics table, not Perf.
If you want to collect Performance counters to Perf table without enabling of VM Insights, please follow the article
https://learn.microsoft.com/en-us/azure/azure-monitor/agents/data-collection-rule-azure-monitor-agent?tabs=portal