`MustParsePromMetrics` imports `lib/protoparser/prometheus`, and this
package exposes the following metrics:
```
vm_protoparser_rows_read_total{type="promscrape"}
vm_rows_invalid_total{type="prometheus"}
```
It means every package that uses `lib/prompbmarshal` will start exposing
these metrics. For example, vlogs imports `lib/protoparser/common` which
uses `lib/prompbmarshal.Label`. And only because of this vlogs starts
exposing unrelated prometheus metrics on /metrics page.
Moving `MustParsePromMetrics` to `lib/protoparser/prometheus` seems like
the leas intrusive change.
-----------
Depends on another change
https://github.com/VictoriaMetrics/VictoriaMetrics/pull/8403
Signed-off-by: hagen1778 <roman@victoriametrics.com>