prometheus: Remove the Metrics class

prometheus: Remove the Metrics class

The class was introduced to account for the 'pending' mechanism but that
is no longer being used by the code so we no longer need it since it
serves a purpose of a simple dictionary, now.

This simplifies the code and brings a bit of performance improvements
since the Metric classes in the collect method are created only if they
did not exist before.

Signed-off-by: Boris Ranto <branto@redhat.com>