this involved adding an optional float value to the ValCount struct
which complicated result types, necessitated grpc changes, and needed
quite a few tests at different layers.
This PR adds a field name (string) to the return types
which represent the values from a specific field. For example,
a TopN query on field `x` would be `TopN(x)` and have results
like:
```
[]Pair{
{ID: 14, Count: 10},
{ID: 3, Count: 8},
{ID: 7, Count: 3},
}
```
In order to know what field this result type refers to, we wrap
`[]Pair` in a new struct called `PairsField` which contains an
addition `Field` string where `x` is stored.
This is useful for informing the gRPC server how to construct
more appropriate headers for the result stream (in this case,
the column headers can now be "x" and "count").
Similar logic was applied to `RowIdentifiers` and `Pair` as well.
add makeRows() tests
register the gRPC server
use api.Index() instead of api.Schema()
support most field types in Inspect() query
currently, there's no support for `time` fields.
those will be dependent upon the output format
and the ability to materialize the timestamp from
the time views.
this commit also changes the response type of the
`Inspect()` query to be a tabular `RowResponse`.
