Explore: Metrics chooser for prometheus

- load all histogrammable metrics on start, based on `{le!=''}` series
  query
- select dropdown shows all those metrics as well as histograms in a
  special group
- select a metric will write fill the metric in the query box
- if a histogram is chosen, it will write a complete
  `histogram_quantile` query
- added new dependency: rc-cascader

package.json

@@ -166,6 +166,7 @@
     "mousetrap-global-bind": "^1.1.0",
     "prismjs": "^1.6.0",
     "prop-types": "^15.6.0",
+    "rc-cascader": "^0.14.0",
     "react": "^16.2.0",
     "react-dom": "^16.2.0",
     "react-grid-layout": "0.16.6",
@@ -187,4 +188,4 @@
   "resolutions": {
     "caniuse-db": "1.0.30000772"
   }
-}
+}