Resolve any metric row through the metric endpoint

`GET /v0/projects/{project}/metrics/{metric}` resolved through the
`metric_boundary` view, which drives on `name = 'value'`, so the UUID of a
`lower_value`, an `upper_value`, or any other named scalar was a 404 even though
the report response hands those UUIDs out. The lookup moves onto the `metric`
table, so every metric row UUID resolves.

The response now describes the addressed row. It gains `name` and `value`, the
addressed row's own name and scalar, and `parameter`, the grid point it was
measured under. `metric` becomes optional and is present only when the addressed
row is the `value` row: reconstructing the triple around a bound or a named
scalar would assert numbers the address does not name. `threshold`, `boundary`,
and `alert` describe the boundary attached to the addressed row itself.

A `value` row address is unchanged but for those three additions, which a
fixture pins key by key.

Author: epompeii