fix(prometheus): use time independent API to list metrics and labels names

Using the "/api/v1/query" endpoint to extract information about metrics
and labels are limited to the metrics available at the time parameter
(that is set to current time), this can lead to labels not showing
because they have no value in the current time even when the dashboard
is displaying historic data.

On the other hand "/api/v1/series" returns results including every
metric and label known to Prometheus, independent of time and value.

public/app/plugins/datasource/prometheus/datasource.js

@@ -148,20 +148,13 @@ function (angular, _, moment, dateMath) {
             });
           });
         } else {
-          var metric_query = 'count(' + label_values_query[1] + ') by (' +
-                             label_values_query[2]  + ')';
-          url = '/api/v1/query?query=' + encodeURIComponent(metric_query) +
-                    '&time=' + (moment().valueOf() / 1000);
+          url = '/api/v1/series?match[]=' + encodeURIComponent(label_values_query[1]);
 
           return this._request('GET', url)
             .then(function(result) {
-              if (result.data.data.result.length === 0 ||
-                  _.keys(result.data.data.result[0].metric).length === 0) {
-                return [];
-              }
-              return _.map(result.data.data.result, function(metricValue) {
+              return _.map(result.data.data, function(metric) {
                 return {
-                  text: metricValue.metric[label_values_query[2]],
+                  text: metric[label_values_query[2]],
                   expandable: true
                 };
               });
@@ -190,14 +183,13 @@ function (angular, _, moment, dateMath) {
           });
       } else {
         // if query contains full metric name, return metric name and label list
-        url = '/api/v1/query?query=' + encodeURIComponent(interpolated) +
-              '&time=' + (moment().valueOf() / 1000);
+        url = '/api/v1/series?match[]=' + encodeURIComponent(interpolated);
 
         return this._request('GET', url)
           .then(function(result) {
-            return _.map(result.data.data.result, function(metricData) {
+            return _.map(result.data.data, function(metric) {
               return {
-                text: getOriginalMetricName(metricData.metric),
+                text: getOriginalMetricName(metric),
                 expandable: true
               };
             });

public/app/plugins/datasource/prometheus/specs/datasource_specs.ts

@@ -59,19 +59,16 @@ describe('PrometheusDatasource', function() {
       ctx.$rootScope.$apply();
       expect(results.length).to.be(3);
     });
-    it('label_values(metric, resource) should generate count metric query', function() {
+    it('label_values(metric, resource) should generate series query', function() {
       response = {
         status: "success",
-        data: {
-          resultType: "vector",
-          result: [
-            {metric: {resource: "value1"}, value: []},
-            {metric: {resource: "value2"}, value: []},
-            {metric: {resource: "value3"}, value: []}
+        data: [
+          {__name__: "metric", resource: "value1"},
+          {__name__: "metric", resource: "value2"},
+          {__name__: "metric", resource: "value3"}
         ]
-        }
       };
-      ctx.$httpBackend.expect('GET', /proxied\/api\/v1\/query\?query=count\(metric\)%20by%20\(resource\)&time=.*/).respond(response);
+      ctx.$httpBackend.expect('GET', 'proxied/api/v1/series?match[]=metric').respond(response);
       ctx.ds.metricFindQuery('label_values(metric, resource)').then(function(data) { results = data; });
       ctx.$httpBackend.flush();
       ctx.$rootScope.$apply();
@@ -90,4 +87,3 @@ describe('PrometheusDatasource', function() {
     });
   });
 });
-