Monitoring metric set size

## Is your feature request related to a problem?

We're running into the default `metric_set_limit` (#2148, #2717) (probably due to many different transaction names). We can obviously increase the limit now based on a good guess, but it doesn't seem possible to examine the current metric set size.

We'd like to be able to monitor the metric set size to ensure we're well below the limit, but still keep the limit rather low to prevent explosion.

## Describe the solution you'd like

It'd be great if we could add a micrometer gauge that monitors the metric set size, or if this becomes a built-in metric.

## Describe alternatives you've considered

Raising the limit higher than necessary.