net: prometheus: Add more parameters to metric macros

Add collector parameter to metric creation macros so that it
is possible to bind the metric to collector already at built
time.

Also add optional user_data to metric macro calls so that user
can add optional data there. This will be used by network statistics
Prometheus support in subsequent commits.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>