nexus: Remove TimeseriesKey from timeseries output

[wfchandler]

Our external API timeseries endpoints currently return data as a BTreeMap<TimeseriesKey, Timeseries>, which results in JSON output with arbitrary numeric keys that have no meaning to API consumers:

{
  "timeseries": {
    "2352746367989923131": { ... },
    "3940108470521992408": { ... }
  }
}

This commit introduces a new TableOutput struct that presents timeseries data as an array instead of a map. The TableOutput type is converted from the internal Table representation at the API boundary, preserving the ordering from the original BTreeMap while providing a cleaner JSON structure:

{
  "timeseries": [ { ... }, { ... } ]
}

Closes #8108

[david-crespo]

Love the idea, TableOutput is kind of a meh name. By “output” do you just mean “view”? Kinda redundant. I feel like it could just be called Table in that case. Might be cool to add another word that indicates it’s about metrics. Do we already have an OxqlResult in the views? Looks like no.

Edit: I see now it’s inside OxqlQueryResult. In that case I think I prefer Table over TableOutput, though neither is a great name for an array of timeseries.

[wfchandler]

Love the idea, TableOutput is kind of a meh name.

@david-crespo agreed. Just Table is an existing type which is still used internally by oximeter, what do you think about MetricsTable?

[david-crespo]

MetricsTable is fine, or you could make Table work by defining it in views.rs? There are plenty of examples where the view struct has the same name as the underlying thing.

[wfchandler]

FYI @askfongjojo, this will slightly change how we parse OxQL results. I'll update the fmt-query.py script in the docs and your JSON metrics exporter.

/cc @jmcarp I think you've been working on tooling for our metrics.

[wfchandler]

Hmm, actually I think the metrics exporter will handle this without change as it uses .timeseries.*.