encapsulate target metadata api (#590)

Signed-off-by: YaoZengzeng <yaozengzeng@zju.edu.cn>

api/prometheus/v1/api.go

@@ -128,6 +128,7 @@ const (
 	epLabelValues     = apiPrefix + "/label/:name/values"
 	epSeries          = apiPrefix + "/series"
 	epTargets         = apiPrefix + "/targets"
+	epTargetsMetadata = apiPrefix + "/targets/metadata"
 	epRules           = apiPrefix + "/rules"
 	epSnapshot        = apiPrefix + "/admin/tsdb/snapshot"
 	epDeleteSeries    = apiPrefix + "/admin/tsdb/delete_series"
@@ -151,6 +152,9 @@ type RuleType string
 // RuleHealth models the health status of a rule.
 type RuleHealth string
 
+// MetricType models the type of a metric.
+type MetricType string
+
 const (
 	// Possible values for AlertState.
 	AlertStateFiring   AlertState = "firing"
@@ -179,6 +183,16 @@ const (
 	RuleHealthGood    = "ok"
 	RuleHealthUnknown = "unknown"
 	RuleHealthBad     = "err"
+
+	// Possible values for MetricType
+	MetricTypeCounter        MetricType = "counter"
+	MetricTypeGauge          MetricType = "gauge"
+	MetricTypeHistogram      MetricType = "histogram"
+	MetricTypeGaugeHistogram MetricType = "gaugehistogram"
+	MetricTypeSummary        MetricType = "summary"
+	MetricTypeInfo           MetricType = "info"
+	MetricTypeStateset       MetricType = "stateset"
+	MetricTypeUnknown        MetricType = "unknown"
 )
 
 // Error is an error returned by the API.
@@ -242,6 +256,8 @@ type API interface {
 	Rules(ctx context.Context) (RulesResult, api.Error)
 	// Targets returns an overview of the current state of the Prometheus target discovery.
 	Targets(ctx context.Context) (TargetsResult, api.Error)
+	// TargetsMetadata returns metadata about metrics currently scraped by the target.
+	TargetsMetadata(ctx context.Context, matchTarget string, metric string, limit string) ([]MetricMetadata, api.Error)
 }
 
 // AlertsResult contains the result from querying the alerts endpoint.
@@ -351,6 +367,15 @@ type DroppedTarget struct {
 	DiscoveredLabels map[string]string `json:"discoveredLabels"`
 }
 
+// MetricMetadata models the metadata of a metric.
+type MetricMetadata struct {
+	Target map[string]string `json:"target"`
+	Metric string            `json:"metric,omitempty"`
+	Type   MetricType        `json:"type"`
+	Help   string            `json:"help"`
+	Unit   string            `json:"unit"`
+}
+
 // queryResult contains result data for a query.
 type queryResult struct {
 	Type   model.ValueType `json:"resultType"`
@@ -760,6 +785,31 @@ func (h *httpAPI) Targets(ctx context.Context) (TargetsResult, api.Error) {
 	return res, api.NewErrorAPI(err, nil)
 }
 
+func (h *httpAPI) TargetsMetadata(ctx context.Context, matchTarget string, metric string, limit string) ([]MetricMetadata, api.Error) {
+	u := h.client.URL(epTargetsMetadata, nil)
+	q := u.Query()
+
+	q.Set("match_target", matchTarget)
+	q.Set("metric", metric)
+	q.Set("limit", limit)
+
+	u.RawQuery = q.Encode()
+
+	req, err := http.NewRequest(http.MethodGet, u.String(), nil)
+	if err != nil {
+		return nil, api.NewErrorAPI(err, nil)
+	}
+
+	_, body, apiErr := h.client.Do(ctx, req)
+	if apiErr != nil {
+		return nil, apiErr
+	}
+
+	var res []MetricMetadata
+	err = json.Unmarshal(body, &res)
+	return res, api.NewErrorAPI(err, nil)
+}
+
 // apiClient wraps a regular client and processes successful API responses.
 // Successful also includes responses that errored at the API level.
 type apiClient struct {

api/prometheus/v1/api_test.go

@@ -174,6 +174,12 @@ func TestAPIs(t *testing.T) {
 		}
 	}
 
+	doTargetsMetadata := func(matchTarget string, metric string, limit string) func() (interface{}, api.Error) {
+		return func() (interface{}, api.Error) {
+			return promAPI.TargetsMetadata(context.Background(), matchTarget, metric, limit)
+		}
+	}
+
 	queryTests := []apiTest{
 		{
 			do: doQuery("2", testTime),
@@ -296,7 +302,7 @@ func TestAPIs(t *testing.T) {
 				"end":   []string{testTime.Format(time.RFC3339Nano)},
 			},
 			res: []model.LabelSet{
-				model.LabelSet{
+				{
 					"__name__": "up",
 					"job":      "prometheus",
 					"instance": "localhost:9090",
@@ -645,6 +651,52 @@ func TestAPIs(t *testing.T) {
 			inErr:     fmt.Errorf("some error"),
 			err:       fmt.Errorf("some error"),
 		},
+
+		{
+			do: doTargetsMetadata("{job=\"prometheus\"}", "go_goroutines", "1"),
+			inRes: []map[string]interface{}{
+				{
+					"target": map[string]interface{}{
+						"instance": "127.0.0.1:9090",
+						"job":      "prometheus",
+					},
+					"type": "gauge",
+					"help": "Number of goroutines that currently exist.",
+					"unit": "",
+				},
+			},
+			reqMethod: "GET",
+			reqPath:   "/api/v1/targets/metadata",
+			reqParam: url.Values{
+				"match_target": []string{"{job=\"prometheus\"}"},
+				"metric":       []string{"go_goroutines"},
+				"limit":        []string{"1"},
+			},
+			res: []MetricMetadata{
+				{
+					Target: map[string]string{
+						"instance": "127.0.0.1:9090",
+						"job":      "prometheus",
+					},
+					Type: "gauge",
+					Help: "Number of goroutines that currently exist.",
+					Unit: "",
+				},
+			},
+		},
+
+		{
+			do:        doTargetsMetadata("{job=\"prometheus\"}", "go_goroutines", "1"),
+			inErr:     fmt.Errorf("some error"),
+			reqMethod: "GET",
+			reqPath:   "/api/v1/targets/metadata",
+			reqParam: url.Values{
+				"match_target": []string{"{job=\"prometheus\"}"},
+				"metric":       []string{"go_goroutines"},
+				"limit":        []string{"1"},
+			},
+			err: fmt.Errorf("some error"),
+		},
 	}
 
 	var tests []apiTest