Support more types of custom metrics

[adriangonz]

As a follow up to #1020, it would be good to support other types of metrics under the "simplified" interface (e.g Counter, Summary, Gauge, etc.). Or, alternatively, under our own version of the metric wrappers which allows for direct access to the metric objects (this would be required to label the metrics accordingly with the right model name / version).

This would also allow for further customisation of the metrics (e.g. changing the buckets in a Histogram).

Originally posted by @sashman in https://github.com/SeldonIO/MLServer/issues/1020#issuecomment-1453180370