Scale transaction registration metric by registered voters

We have a counter metric that tracks how many transactions have been registered, started, committed or aborted/failed. But while registration happens at node-manager level and thus happens once, only, the other actions will happen at voter level and thus happen as many times as there are voters. This creates a discrepancy in observed metrics, e.g. in the case where we have three voters, we'd see one registration but three started/committed transactions, making it hard to correlate these two.

While the registration counter could just be scaled at dashboard level, one would need to know how many nodes are going to take part in each transaction. As this isn't generic information and may also change at runtime (e.g. because one node goes down meanwhile), doing so statically doesn't help at all. So let's instead just scale the registration counter in code to make the metric more useful.

Fixes #2957 (closed)