Add /labels API to client (#604)

API ref https://prometheus.io/docs/prometheus/latest/querying/api/#getting-label-names

Signed-off-by: Thomas Jackson <jacksontj.89@gmail.com>

api/prometheus/v1/api.go

@@ -124,6 +124,7 @@ const (
 	epAlertManagers   = apiPrefix + "/alertmanagers"
 	epQuery           = apiPrefix + "/query"
 	epQueryRange      = apiPrefix + "/query_range"
+	epLabels          = apiPrefix + "/labels"
 	epLabelValues     = apiPrefix + "/label/:name/values"
 	epSeries          = apiPrefix + "/series"
 	epTargets         = apiPrefix + "/targets"
@@ -227,6 +228,8 @@ type API interface {
 	DeleteSeries(ctx context.Context, matches []string, startTime time.Time, endTime time.Time) error
 	// Flags returns the flag values that Prometheus was launched with.
 	Flags(ctx context.Context) (FlagsResult, error)
+	// LabelNames returns all the unique label names present in the block in sorted order.
+	LabelNames(ctx context.Context) ([]string, error)
 	// LabelValues performs a query for the values of the given label.
 	LabelValues(ctx context.Context, label string) (model.LabelValues, error)
 	// Query performs a query for the given time.
@@ -622,6 +625,20 @@ func (h *httpAPI) Flags(ctx context.Context) (FlagsResult, error) {
 	return res, json.Unmarshal(body, &res)
 }
 
+func (h *httpAPI) LabelNames(ctx context.Context) ([]string, error) {
+	u := h.client.URL(epLabels, nil)
+	req, err := http.NewRequest(http.MethodGet, u.String(), nil)
+	if err != nil {
+		return nil, err
+	}
+	_, body, _, err := h.client.Do(ctx, req)
+	if err != nil {
+		return nil, err
+	}
+	var labelNames []string
+	return labelNames, json.Unmarshal(body, &labelNames)
+}
+
 func (h *httpAPI) LabelValues(ctx context.Context, label string) (model.LabelValues, error) {
 	u := h.client.URL(epLabelValues, map[string]string{"name": label})
 	req, err := http.NewRequest(http.MethodGet, u.String(), nil)

api/prometheus/v1/api_test.go

@@ -135,6 +135,13 @@ func TestAPIs(t *testing.T) {
 		}
 	}
 
+	doLabelNames := func(label string) func() (interface{}, api.Warnings, error) {
+		return func() (interface{}, api.Warnings, error) {
+			v, err := promAPI.LabelNames(context.Background())
+			return v, nil, err
+		}
+	}
+
 	doLabelValues := func(label string) func() (interface{}, api.Warnings, error) {
 		return func() (interface{}, api.Warnings, error) {
 			v, err := promAPI.LabelValues(context.Background(), label)
@@ -324,6 +331,22 @@ func TestAPIs(t *testing.T) {
 			err: fmt.Errorf("some error"),
 		},
 
+		{
+			do:        doLabelNames("mylabel"),
+			inRes:     []string{"val1", "val2"},
+			reqMethod: "GET",
+			reqPath:   "/api/v1/labels",
+			res:       []string{"val1", "val2"},
+		},
+
+		{
+			do:        doLabelNames("mylabel"),
+			inErr:     fmt.Errorf("some error"),
+			reqMethod: "GET",
+			reqPath:   "/api/v1/labels",
+			err:       fmt.Errorf("some error"),
+		},
+
 		{
 			do:        doLabelValues("mylabel"),
 			inRes:     []string{"val1", "val2"},