Overhaul stats: Remove internal non-labeled metrics

[josecelano]

Depends on: torrust/torrust-demo#63

After implementing #1446 we don't need anymore the global metrics. All metrics internally are labeled metrics.

The API builds the global metrics for the stats endpoint from the labeled metrics.

Before implementing this issue, confirm that the stats endpoint is correct. The verification process is described in this issue.

The stats endpoint: http://localhost:1212/api/v1/stats?token=MyAccessToken&format=prometheus