Add global ordinal info to stats APIs. (#94500)

This change adds:
* Total global ordinal build time for all fields and per field.
* Max shard value count per field. The value count is per shard and of the shard with the highest count. Reporting value on index level or across indices is too expensive to report or keep track of.

This is added to common stats, which
is exposed in several stats APIs.

The following api call:

```
GET /_nodes/stats?filter_path=nodes.*.indices.fielddata&fields=key,key2
```

Returns:

```
{
    "nodes": {
        "pcMNy4GsQ8ef6Rw-bI2EFg": {
            "indices": {
                "fielddata": {
                    "memory_size_in_bytes": 2552,
                    "evictions": 0,
                    "fields": {
                        "key2": {
                            "memory_size_in_bytes": 1320
                        },
                        "key": {
                            "memory_size_in_bytes": 1232
                        }
                    },
                    "global_ordinals": {
                        "build_time_in_millis": 8,
                        "fields": {
                            "key2": {
                                "build_time_in_millis": 4,
                                "shard_max_value_count": 4
                            },
                            "key": {
                                "build_time_in_millis": 4,
                                "shard_max_value_count": 4
                            }
                        }
                    }
                }
            }
        }
    }
}
```

docs/changelog/94500.yaml

@@ -0,0 +1,5 @@
+pr: 94500
+summary: Add global ordinal info to stats APIs
+area: Aggregations
+type: enhancement
+issues: []

docs/reference/cluster/stats.asciidoc

@@ -285,6 +285,26 @@ shards assigned to selected nodes.
 (integer)
 Total number of evictions from the field data cache across all shards assigned
 to selected nodes.
+
+`global_ordinals.build_time`::
+(<<time-units,time unit>>)
+The total time spent building global ordinals for all fields.
+
+`global_ordinals.build_time_in_millis`::
+(integer)
+The total time, in milliseconds, spent building global ordinals for all fields.
+
+`global_ordinals.fields.[field-name].build_time`::
+(<<time-units,time unit>>)
+The total time spent building global ordinals for field with specified name.
+
+`global_ordinals.fields.[field-name].build_time_in_millis`::
+(integer)
+The total time, in milliseconds, spent building global ordinals for field with specified name.
+
+`global_ordinals.fields.[field-name].shard_max_value_count`::
+(long)
+The total time spent building global ordinals for field with specified name.
 =====
 
 `query_cache`::
@@ -1514,7 +1534,11 @@ The API returns the following response:
       "fielddata": {
          "memory_size": "0b",
          "memory_size_in_bytes": 0,
-         "evictions": 0
+         "evictions": 0,
+         "global_ordinals": {
+            "build_time" : "0s",
+            "build_time_in_millis" : 0
+         }
       },
       "query_cache": {
          "memory_size": "0b",