Use the interface type `RoundTripper` in `FetchMetricFamilies`
Use the interface type `RoundTripper` in `FetchMetricFamilies`

If using `Transport`, the `nil` detection doesn't work anymore.

Signed-off-by: beorn7 <a31805f6001ab3b535ea95133fbc4db46e97e8d1@rabenste.in>

prometheus/prom2json