Add ClickHouse http client timeout (#27)
Add ClickHouse http client timeout (#27)

30 seconds for each request is long enough for ClickHouse to
respond under any conditions, but short enough to close stuck
connections.

Making it configurable means breaking `NewExporter` clients.
Or, a new interface for creating exporters has to be created.

Closes #25 which is most likely caused by
https://github.com/yandex/ClickHouse/issues/4060.
f1yegor/clickhouse_exporter