Prometheus metric names, types and units could be cleaner and more consistent

Just some observations, there are probably more:

- `rotonda_task_total_poll_count_total` - contains the term `total` twice and also `count`.
- `rotonda_rib_merge_update_announce_duration_microseconds_count` - why does this `count` not end in `_total` (the metric type) while the one above does, do they have a different rotonda/rtrtr `MetricUnit` ?