Change GeoHashGrid.Bucket#getKey() to return String

The GeoHashGrid bucket class currently returns a GeoPoint with its getKey()
method. The REST API returns geohash as a String in the key field of the bucket.
This changes the getKey() method to also return a String. Having the ability to
get a GeoPoint from a bucket that represents a geohash cell is misleading
because the cell does not represent a point but an area. Instead, it should be
the clients responsibility to make the decision on how to represent the bucket
key in its application. This also simplifies implementing the high level rest
client aspects of #30320, as the client will not need to know about the
GeoHashType being used and will only care that there is some string key for the
buckets.

Relates to #30320

Author: Christoph Büscher

docs/reference/migration/migrate_7_0/aggregations.asciidoc

@@ -23,3 +23,10 @@ Aggregation is now a variable called `state` available in the script context, ra
 being provided via the `params` object as `params._agg`.
 
 The old `params._agg` variable is still available as well.
+
+==== Change GeoHashGrid.Bucket#getKey() to return a String instead of a GeoPoint
+
+The aggregation bucket used to return a GeoPoint representing the bucket keys hash
+value. This was not consistent with the Rest API output that represents the key simply
+as the string value of the geohash. The Java API now also returns a String here which
+can be converted to a point as needed by the client.

server/src/main/java/org/elasticsearch/common/geo/GeoHashUtils.java

@@ -77,7 +77,7 @@ private static long longEncode(final String hash, int length) {
         long b;
         long l = 0L;
         for(char c : hash.toCharArray()) {
-            b = (long)(BASE_32_STRING.indexOf(c));
+            b = BASE_32_STRING.indexOf(c);
             l |= (b<<(level--*5));
             if (level < 0) {
                 // We cannot handle more than 12 levels
@@ -178,7 +178,7 @@ public static final long mortonEncode(final String hash) {
         long b;
         long l = 0L;
         for(char c : hash.toCharArray()) {
-            b = (long)(BASE_32_STRING.indexOf(c));
+            b = BASE_32_STRING.indexOf(c);
             if (b < 0) {
                 throw new IllegalArgumentException("unsupported symbol [" + c + "] in geohash [" + hash + "]");
             }

server/src/main/java/org/elasticsearch/search/aggregations/bucket/geogrid/InternalGeoHashGrid.java

@@ -20,7 +20,6 @@
 
 import org.apache.lucene.util.PriorityQueue;
 import org.elasticsearch.common.geo.GeoHashUtils;
-import org.elasticsearch.common.geo.GeoPoint;
 import org.elasticsearch.common.io.stream.StreamInput;
 import org.elasticsearch.common.io.stream.StreamOutput;
 import org.elasticsearch.common.util.LongObjectPagedHashMap;
@@ -77,12 +76,12 @@ public void writeTo(StreamOutput out) throws IOException {
 
         @Override
         public String getKeyAsString() {
-            return GeoHashUtils.stringEncode(geohashAsLong);
+            return getKey();
         }
 
         @Override
-        public GeoPoint getKey() {
-            return GeoPoint.fromGeohash(geohashAsLong);
+        public String getKey() {
+            return GeoHashUtils.stringEncode(geohashAsLong);
         }
 
         @Override

server/src/main/java/org/elasticsearch/search/aggregations/bucket/geogrid/ParsedGeoHashGrid.java

@@ -19,7 +19,6 @@
 
 package org.elasticsearch.search.aggregations.bucket.geogrid;
 
-import org.elasticsearch.common.geo.GeoPoint;
 import org.elasticsearch.common.xcontent.ObjectParser;
 import org.elasticsearch.common.xcontent.XContentBuilder;
 import org.elasticsearch.common.xcontent.XContentParser;
@@ -57,8 +56,8 @@ public static class ParsedBucket extends ParsedMultiBucketAggregation.ParsedBuck
         private String geohashAsString;
 
         @Override
-        public GeoPoint getKey() {
-            return GeoPoint.fromGeohash(geohashAsString);
+        public String getKey() {
+            return geohashAsString;
         }
 
         @Override