Generate key for ip range from/to agg

Elasticsearch 5.0 no longer returns a `key` prop with ip range buckets
when from/to is used in the request. Looking at the [2.x docs][1] it
appears to be a mistake that it was ever included in the first place.
So now we'll generate the key ourselves.

[1]: https://www.elastic.co/guide/en/elasticsearch/reference/2.4/search-aggregations-bucket-iprange-aggregation.html

Fixes: elastic#8736

src/ui/public/agg_types/buckets/ip_range.js

@@ -13,6 +13,13 @@ export default function RangeAggDefinition(Private) {
     name: 'ip_range',
     title: 'IPv4 Range',
     createFilter: createFilter,
+    getKey: function (bucket, key, agg) {
+      if (key) return key;
+
+      const from = _.get(bucket, 'from', '*');
+      const to = _.get(bucket, 'to', '*');
+      return `${from}-${to}`;
+    },
     makeLabel: function (aggConfig) {
       return aggConfig.getFieldDisplayName() + ' IP ranges';
     },