Oximeter targets and metrics need versions

[bnaecker]

The oximeter::Target and oximeter::Metric traits need to support a version number for each. Ideally, this would be easily expressed in the derive macro. The actual timeseries for a target/metric pair also needs to be versioned, but this should be derived from the version of those parts. E.g., <target_version>.<metric_version>.

This should also be included in the database representation of the timeseries schema. This will interact with the current Nexus API endpoint for listing the schema, though that may just disappear at some point.

[bnaecker]

I've posted a PR which adds versions to the target / metric types themselves. The next chunk is to read / write them in the DB when doing both insertions or queries. That will involve some tooling to update the ClickHouse schema as well, since we'll want to add columns to the timeseries_schema table to track them.

[bnaecker]

Completed in #5889