[oximeter] Modify to write to both single node and replicated cluster

As part of https://github.com/oxidecomputer/omicron/issues/7338 we need oximeter to temporarily write to both clickhouse installations at the same time. This functionality is necessary for stage one of [RFD 468](https://rfd.shared.oxide.computer/rfd/0468). We will be removing this functionality once we have completed the roll out of replicated ClickHouse.