increase limit to lookup unique metric tag values

Default limit is 25 which is too small. Considering currently the
/api/search/lookup query isn't narrowed down by selected tag
keys and values (see https://github.com/grafana/grafana/pull/1433), 
the limit is set to 3000, this should be fine because people rarely
create new graph panel.

public/app/plugins/datasource/opentsdb/datasource.js

@@ -93,7 +93,7 @@ function (angular, _, kbn) {
 
       var m = metric + "{" + key + "=*}";
 
-      return this._get('/api/search/lookup', {m: m}).then(function(result) {
+      return this._get('/api/search/lookup', {m: m, limit: 3000}).then(function(result) {
         result = result.data.results;
         var tagvs = [];
         _.each(result, function(r) {