Explain how to enable/disable asynchronous_metric_log

[gingerwizard]

Two issues on the documentation page: https://clickhouse.com/docs/en/operations/system-tables/asynchronous_metric_log

1. First it says it's enabled by default. Which does not seem to be the case.
2. We need document how to enable/disable it.

cc @lio-p

[gingerwizard]

Done