Allow disabling the built-in Go Metrics

[dalegaard]

The golang Prometheus library exposes a bunch of Go-specific metrics in the default registry, but these don't really have anything to do with Clickhouse.

This commit adds an option to disable these default metrics, by way of creating a new registry instead.

To preserve backwards compatibility, the option is disabled by default and must be explicit toggled on using the -clickhouse_only flag.

[f1yegor]

Merged in master, thank for your contribution.

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