Simplify oxql responses

Here's an excerpt of the response from the timeseries query endpoint:

            "values": [
              {
                "metric_type": "delta",
                "values": {
                  "type": "integer",
                  "values": [
                    118824099830713,
                    4957551326,
                    4966427295,
                    4956182957,
                    4958472616
                  ]
                }
              }
            ]

I think this can be simpler in a few ways:

• The metric and value types are properties of the metric, so I think we can hoist them to the level of the table, rather than nesting them and repeating them for each timeseries.
• We have a top-level values array, which includes a values map, which includes another values array. Can we either elide some of this nesting (if we move metric_type and type to a higher level, this structure would only include fields named "values"), or pick more meaningful names? And out of curiosity, will the top-level values array ever have more than one element?

Probably questions for @bnaecker.

Start with the timeseries query endpoint and the response example in this issue. Resolve whether metric and value types can move to table level, whether the nested values fields should be renamed or removed, and whether the top-level array can contain multiple elements. Done means the response shape and its intended cardinality are agreed and documented for implementation.