add tests for log volume middleware (#11126)

**What this PR does / why we need it**:
 
This adds some tests around the volume middleware, which were very
useful in understanding a recent issue

pkg/querier/queryrange/volume_test.go

@@ -1,15 +1,19 @@
 package queryrange
 
 import (
+	"context"
 	"testing"
 	"time"
 
 	"github.com/stretchr/testify/require"
 
+	"github.com/grafana/dskit/user"
+
 	"github.com/grafana/loki/pkg/loghttp"
 	"github.com/grafana/loki/pkg/logproto"
 	"github.com/grafana/loki/pkg/push"
 	"github.com/grafana/loki/pkg/querier/queryrange/queryrangebase"
+	"github.com/grafana/loki/pkg/storage/stores/index/seriesvolume"
 )
 
 const forRangeQuery = false
@@ -250,3 +254,78 @@ func Test_toPrometheusResponse(t *testing.T) {
 		}, promResp.Response.Data)
 	})
 }
+
+func Test_VolumeMiddleware(t *testing.T) {
+	makeVolumeRequest := func(req *logproto.VolumeRequest) *queryrangebase.PrometheusResponse {
+		nextHandler := queryrangebase.HandlerFunc(func(ctx context.Context, r queryrangebase.Request) (queryrangebase.Response, error) {
+			return &VolumeResponse{
+				Response: &logproto.VolumeResponse{
+					Volumes: []logproto.Volume{
+						{
+							Name:   `{foo="bar"}`,
+							Volume: 42,
+						},
+					},
+				},
+			}, nil
+		})
+
+		m := NewVolumeMiddleware()
+		wrapped := m.Wrap(nextHandler)
+
+		ctx := user.InjectOrgID(context.Background(), "fake")
+		resp, err := wrapped.Do(ctx, req)
+		require.NoError(t, err)
+		require.NotNil(t, resp)
+
+		return resp.(*LokiPromResponse).Response
+	}
+
+	t.Run("it breaks query up into subqueries according to step", func(t *testing.T) {
+		volumeReq := &logproto.VolumeRequest{
+			From:        10,
+			Through:     20,
+			Matchers:    `{foo="bar"}`,
+			Limit:       seriesvolume.DefaultLimit,
+			Step:        1,
+			AggregateBy: seriesvolume.Series,
+		}
+		promResp := makeVolumeRequest(volumeReq)
+
+		require.Equal(t, promResp.Data.ResultType, loghttp.ResultTypeMatrix)
+		require.Equal(t, len(promResp.Data.Result), 1)
+		require.Equal(t, len(promResp.Data.Result[0].Samples), 10)
+	})
+
+	t.Run("only returns one datapoint when step is > than time range", func(t *testing.T) {
+		volumeReq := &logproto.VolumeRequest{
+			From:        10,
+			Through:     20,
+			Matchers:    `{foo="bar"}`,
+			Limit:       seriesvolume.DefaultLimit,
+			Step:        20,
+			AggregateBy: seriesvolume.Series,
+		}
+		promResp := makeVolumeRequest(volumeReq)
+
+		require.Equal(t, promResp.Data.ResultType, loghttp.ResultTypeVector)
+		require.Equal(t, len(promResp.Data.Result), 1)
+		require.Equal(t, len(promResp.Data.Result[0].Samples), 1)
+	})
+
+	t.Run("when requested time range is not evenly divisible by step, an extra datpoint is added", func(t *testing.T) {
+		volumeReq := &logproto.VolumeRequest{
+			From:        1698830441000, // 2023-11-01T09:20:41Z
+			Through:     1698830498000, // 2023-11-01T09:21:38Z, difference is 57s
+			Matchers:    `{foo="bar"}`,
+			Limit:       seriesvolume.DefaultLimit,
+			Step:        60000, // 60s
+			AggregateBy: seriesvolume.Series,
+		}
+		promResp := makeVolumeRequest(volumeReq)
+
+		require.Equal(t, promResp.Data.ResultType, loghttp.ResultTypeMatrix)
+		require.Equal(t, 1, len(promResp.Data.Result))
+		require.Equal(t, 2, len(promResp.Data.Result[0].Samples))
+	})
+}