Address comments

Signed-off-by: Fabian Reinartz <freinartz@google.com>

docs/querying/api.md

@@ -363,9 +363,10 @@ $ curl http://localhost:9090/api/v1/targets
 }
 ```
 
-## Target metadata
+## Querying target metadata
 
 The following endpoint returns metadata about metrics currently scraped by targets.
+This is **experimental** and might change in the future.
 
 ```
 GET /api/v1/targets/metadata
@@ -373,7 +374,8 @@ GET /api/v1/targets/metadata
 
 URL query parameters:
 
-- `match=<series_selector>`: A selector that matches targets by label matchers. If a metric name is provided, only metadata for that metric name is returned.
+- `match_target=<label_selectors>`: Label selectors that match targets by their label sets. All targets are selected if left empty.
+- `metric=<string>`: A metric name to retrieve metadata for. All metric metadata is retrieved if left empty.
 - `limit=<number>`: Maximum number of targets to match.
 
 The `data` section of the query result consists of a list of objects that
@@ -384,7 +386,8 @@ from the first two targets with label `job="prometheus"`.
 
 ```json
 curl -G http://localhost:9091/api/v1/targets/metadata \
-    --data-urlencode 'match=go_goroutines{job="prometheus"}' \
+    --data-urlencode 'metric=go_goroutines' \
+    --data-urlencode 'match_target={job="prometheus"}' \
     --data-urlencode 'limit=2'
 {
   "status": "success",
@@ -409,12 +412,12 @@ curl -G http://localhost:9091/api/v1/targets/metadata \
 }
 ```
 
-The following example returns metadata for all metrics for the target with
+The following example returns metadata for all metrics for all targets with
 label `instance="127.0.0.1:9090`.
 
 ```json
 curl -G http://localhost:9091/api/v1/targets/metadata \
-    --data-urlencode 'match={instance="127.0.0.1:9090"}'
+    --data-urlencode 'match_target={instance="127.0.0.1:9090"}'
 {
   "status": "success",
   "data": [

scrape/scrape.go

@@ -547,7 +547,7 @@ type scrapeCache struct {
 
 // metaEntry holds meta information about a metric.
 type metaEntry struct {
-	lastIter uint64 // last scrape iteration the entry was observed
+	lastIter uint64 // Last scrape iteration the entry was observed at.
 	typ      textparse.MetricType
 	help     string
 }
@@ -684,10 +684,12 @@ func (c *scrapeCache) getMetadata(metric string) (MetricMetadata, bool) {
 	}, true
 }
 
-func (c *scrapeCache) listMetadata() (res []MetricMetadata) {
+func (c *scrapeCache) listMetadata() []MetricMetadata {
 	c.metaMtx.Lock()
 	defer c.metaMtx.Unlock()
 
+	res := make([]MetricMetadata, 0, len(c.metadata))
+
 	for m, e := range c.metadata {
 		res = append(res, MetricMetadata{
 			Metric: m,

web/api/v1/api.go

@@ -488,29 +488,21 @@ func (api *API) targets(r *http.Request) (interface{}, *apiError, func()) {
 	return res, nil, nil
 }
 
-func (api *API) targetMetadata(r *http.Request) (interface{}, *apiError) {
+func (api *API) targetMetadata(r *http.Request) (interface{}, *apiError, func()) {
 	limit := -1
 	if s := r.FormValue("limit"); s != "" {
 		var err error
 		if limit, err = strconv.Atoi(s); err != nil {
-			return nil, &apiError{errorBadData, fmt.Errorf("limit must be a number")}
+			return nil, &apiError{errorBadData, fmt.Errorf("limit must be a number")}, nil
 		}
 	}
 
-	matchers, err := promql.ParseMetricSelector(r.FormValue("match"))
+	matchers, err := promql.ParseMetricSelector(r.FormValue("match_target"))
 	if err != nil {
-		return nil, &apiError{errorBadData, err}
+		return nil, &apiError{errorBadData, err}, nil
 	}
 
-	var metric string
-	for i, m := range matchers {
-		// Extract metric matcher.
-		if m.Name == labels.MetricName && m.Type == labels.MatchEqual {
-			metric = m.Value
-			matchers = append(matchers[:i], matchers[i+1:]...)
-			break
-		}
-	}
+	metric := r.FormValue("metric")
 
 	var res []metricMetadata
 Outer:
@@ -546,9 +538,9 @@ Outer:
 		}
 	}
 	if len(res) == 0 {
-		return nil, &apiError{errorNotFound, errors.New("specified metadata not found")}
+		return nil, &apiError{errorNotFound, errors.New("specified metadata not found")}, nil
 	}
-	return res, nil
+	return res, nil, nil
 }
 
 type metricMetadata struct {