Remove prometheus.const-label

[lucacome]

As explained in https://prometheus.io/docs/instrumenting/writing_exporters/#target-labels-not-static-scraped-labels we shouldn't add the same label to all the metrics.

If you ever find yourself wanting to apply the same label to all of your metrics, stop.
...
Accordingly, these labels belong up in the scrape configs of Prometheus via whatever service discovery you’re using.

I wanted to check if people use const labels or if it's okay to remove them.