Make it easier to define multiple tests and run them concurrently.

This test verifies that:
- a gauge metric that has been reset is no longer reported;
- a previously-reset metric that is used again correctly reports its
  values.

The metric key index now accepts a function that gets called every time
a key is deleted from the index, which happens at the next sync after
another worker had deleted a key. The callback function finds the same
metric in the local worker's registry and cleans up its lookup table.

To make sure this happens deterministically for all workers, the key
index sync is now also scheduled to run every `sync_interval` (1s
by default).

The goals are:
- to prevent unbounded growth of the per-metric lookup tables;
- to ensure that previously reset metrics can be used again.

Size based cleanup was added to avoid unbounded growth of lookup tables
as metrics get deleted/reset. This should not be necessary now when lookup
tables are being reset on metric deletion.

This reverts eb1876d