api: adds label values query implementation

This commit adds a client side implementation for fetching label values
from prometheus api.

api/prometheus/api.go

@@ -280,8 +280,10 @@ func (qr *queryResult) UnmarshalJSON(b []byte) error {
 type QueryAPI interface {
 	// Query performs a query for the given time.
 	Query(ctx context.Context, query string, ts time.Time) (model.Value, error)
-	// Query performs a query for the given range.
+	// QueryRange performs a query for the given range.
 	QueryRange(ctx context.Context, query string, r Range) (model.Value, error)
+	// QueryLabelValues performs a query for the values of the given label.
+	QueryLabelValues(ctx context.Context, label string) ([]string, error)
 }
 
 // NewQueryAPI returns a new QueryAPI for the client.
@@ -346,3 +348,15 @@ func (h *httpQueryAPI) QueryRange(ctx context.Context, query string, r Range) (m
 
 	return model.Value(qres.v), err
 }
+
+func (h *httpQueryAPI) QueryLabelValues(ctx context.Context, label string) ([]string, error) {
+	u := h.client.url(epLabelValues, map[string]string{"name": label})
+	req, _ := http.NewRequest(http.MethodGet, u.String(), nil)
+	_, body, err := h.client.do(ctx, req)
+	if err != nil {
+		return nil, err
+	}
+	var values []string
+	err = json.Unmarshal(body, &values)
+	return values, err
+}

api/prometheus/api_test.go

@@ -19,6 +19,7 @@ import (
 	"net/http"
 	"net/url"
 	"reflect"
+	"strings"
 	"testing"
 	"time"
 
@@ -313,9 +314,13 @@ type apiTest struct {
 }
 
 func (c *apiTestClient) url(ep string, args map[string]string) *url.URL {
+	path := apiPrefix + ep
+	for k, v := range args {
+		path = strings.Replace(path, ":"+k, v, -1)
+	}
 	u := &url.URL{
 		Host: "test:9090",
-		Path: apiPrefix + ep,
+		Path: path,
 	}
 	return u
 }
@@ -368,6 +373,12 @@ func TestAPIs(t *testing.T) {
 		}
 	}
 
+	doQueryLabelValues := func(label string) func() (interface{}, error) {
+		return func() (interface{}, error) {
+			return queryAPI.QueryLabelValues(context.Background(), label)
+		}
+	}
+
 	queryTests := []apiTest{
 		{
 			do: doQuery("2", testTime),
@@ -421,6 +432,22 @@ func TestAPIs(t *testing.T) {
 			},
 			err: fmt.Errorf("some error"),
 		},
+
+		{
+			do:        doQueryLabelValues("mylabel"),
+			inRes:     []string{"val1", "val2"},
+			reqMethod: "GET",
+			reqPath:   "/api/v1/label/mylabel/values",
+			res:       []string{"val1", "val2"},
+		},
+
+		{
+			do:        doQueryLabelValues("mylabel"),
+			inErr:     fmt.Errorf("some error"),
+			reqMethod: "GET",
+			reqPath:   "/api/v1/label/mylabel/values",
+			err:       fmt.Errorf("some error"),
+		},
 	}
 
 	var tests []apiTest