Bucket Path to access Top-hits aggregation in elasticsearch query

Hello,
I have an index "jmx_metrics-*" that contains JVM monitoring data from Jconsole.
Following it the mapping of index.

{
  "jmx_metrics-2021.03" : {
    "aliases" : { },
    "mappings" : {
      "properties" : {
        "@timestamp" : {
          "type" : "date"
        },
        "@version" : {
          "type" : "text",
          "fields" : {
            "keyword" : {
              "type" : "keyword",
              "ignore_above" : 256
            }
          }
        },
        "JVM_Instance" : {
          "type" : "text",
          "fields" : {
            "keyword" : {
              "type" : "keyword",
              "ignore_above" : 256
            }
          }
        },
        "heapMax" : {
          "type" : "long"
        },
        "heapUsed" : {
          "type" : "long"
        },
        "heap_max" : {
          "type" : "text",
          "fields" : {
            "keyword" : {
              "type" : "keyword",
              "ignore_above" : 256
            }
          }
        },
        "heap_used" : {
          "type" : "text",
          "fields" : {
            "keyword" : {
              "type" : "keyword",
              "ignore_above" : 256
            }
          }
        },
        "host" : {
          "type" : "text",
          "fields" : {
            "keyword" : {
              "type" : "keyword",
              "ignore_above" : 256
            }
          }
        },
        "metric_path" : {
          "type" : "text",
          "fields" : {
            "keyword" : {
              "type" : "keyword",
              "ignore_above" : 256
            }
          }
        },
        "metric_value_number" : {
          "type" : "long"
        },
        "path" : {
          "type" : "text",
          "fields" : {
            "keyword" : {
              "type" : "keyword",
              "ignore_above" : 256
            }
          }
        },
        "tags" : {
          "type" : "text",
          "fields" : {
            "keyword" : {
              "type" : "keyword",
              "ignore_above" : 256
            }
          }
        }
      }
    },
    "settings" : {
      "index" : {
        "opendistro" : {
          "index_state_management" : {
            "policy_id" : "delete_policy"
          }
        },
        "number_of_shards" : "1",
        "provided_name" : "jmx_metrics-2021.03",
        "creation_date" : "1614556822070",
        "number_of_replicas" : "1",
        "uuid" : "vYh1ADkwTVuj9Yj25TbUvw",
        "version" : {
          "created" : "7080099",
          "upgraded" : "7100299"
        }
      }
    }
  }
}

Below is the query I am trying to write.

GET jmx_metrics-*/_search?size=0
{
  "query": {
    "bool": {
      "filter": [
        {
          "terms": {
            "metric_path.keyword": [
              "jmx.Memory.HeapMemoryUsage.max",
              "jmx.Memory.HeapMemoryUsage.used"
            ]
          }
        },
        {
          "range": {
            "@timestamp": {
             "gte": "now-15m",
              "lte": "now"
            }
          }
        }
      ]
    }
  }, 
  "aggs": {
    "jvm_instance": {
      "terms": {
        "field": "JVM_Instance.keyword",
        "size": 4
      },
      "aggs": {
        "heap_used": {
          "top_hits": {
            "size": 5,
            "sort": [
              {
                "@timestamp": {
                  "order": "desc"
                }
              }
            ]
          }
        },
        "filter_used" : {
          "bucket_selector": {
            "buckets_path": {
              "heap_Used" : "***STUCK_HERE***"
            },
            "script": "params.heap_Used > 0", 
            "gap_policy": "skip"
          }
        },
        "heap_max": {
          "top_hits": {
            "size": 5,
            "sort": [
              {
                "@timestamp": {
                  "order": "desc"
                }
              }
            ]
          }
        }
      }
    }
  },
  "size": 0,
  "stored_fields": [
    "*"
  ],
  "script_fields": {},
  "docvalue_fields": [
    {
      "field": "@timestamp",
      "format": "date_time"
    }
  ]
}

In the above query, I want to access "heapUsed" field but I am not able to access it in bucket path.
Below is the response of the query.

{
  "took" : 57,
  "timed_out" : false,
  "_shards" : {
    "total" : 2,
    "successful" : 1,
    "skipped" : 0,
    "failed" : 1,
    "failures" : [
      {
        "shard" : 0,
        "index" : "jmx_metrics-2021.02",
        "node" : "zQkWWSBvR5KRl8yt8R0OnQ",
        "reason" : {
          "type" : "query_shard_exception",
          "reason" : "No mapping found for [heapUsed] in order to sort on",
          "index_uuid" : "GlySyr7dR7qck2xutTMDnA",
          "index" : "jmx_metrics-2021.02"
        }
      }
    ]
  },
  "hits" : {
    "total" : {
      "value" : 240,
      "relation" : "eq"
    },
    "max_score" : null,
    "hits" : [ ]
  },
  "aggregations" : {
    "jvm_instance" : {
      "doc_count_error_upper_bound" : 0,
      "sum_other_doc_count" : 0,
      "buckets" : [
        {
          "key" : "Transaction",
          "doc_count" : 60,
          "heap_used" : {
            "hits" : {
              "total" : {
                "value" : 60,
                "relation" : "eq"
              },
              "max_score" : null,
              "hits" : [
                {
                  "_index" : "jmx_metrics-2021.03",
                  "_type" : "_doc",
                  "_id" : "Cu9BiHgB82vNnaCaGSaG",
                  "_score" : null,
                  "_source" : {
                    "@timestamp" : "2021-03-31T12:27:50.732Z",
                    "@version" : "1",
                    "host" : "1.1.1.1",
                    "type" : null,
                    "heapUsed" : 2947301544,  **<--- I want to access this field in bucket path**
                    "JVM_Instance" : "Transaction",
                    "path" : "/etc/logstash/conf.d/jmx/Transaction",
                    "metric_value_number" : 2947301544,
                    "metric_path" : "jmx.Memory.HeapMemoryUsage.used",
                    "tags" : [
                      "jmx",
                      "Transaction"
                    ]
                  },
                  "sort" : [
                    1617193670732,
                    2947301544
                  ]
                },
                {
                  "_index" : "jmx_metrics-2021.03",
                  "_type" : "_doc",
                  "_id" : "Z-9BiHgB82vNnaCaGSaG",
                  "_score" : null,
                  "_source" : {
                    "@timestamp" : "2021-03-31T12:27:50.732Z",
                    "@version" : "1",
                    "host" : "1.1.1.1",
                    "type" : null,
                    "JVM_Instance" : "Transaction",
                    "heapMax" : 10468982784,
                    "path" : "/etc/logstash/conf.d/jmx/Transaction",
                    "metric_value_number" : 10468982784,
                    "metric_path" : "jmx.Memory.HeapMemoryUsage.max",
                    "tags" : [
                      "jmx",
                      "Transaction"
                    ]
                  },
                  "sort" : [
                    1617193670732,
                    -9223372036854775808
                  ]
                },
                {
                  "_index" : "jmx_metrics-2021.03",
                  "_type" : "_doc",
                  "_id" : "C-9BiHgB82vNnaCaGSaG",
                  "_score" : null,
                  "_source" : {
                    "@timestamp" : "2021-03-31T12:27:50.731Z",
                    "@version" : "1",
                    "host" : "1.1.1.1",
                    "type" : null,
                    "heapUsed" : 2947256488,
                    "JVM_Instance" : "Transaction",
                    "path" : "/etc/logstash/conf.d/jmx/Transaction",
                    "metric_value_number" : 2947256488,
                    "metric_path" : "jmx.Memory.HeapMemoryUsage.used",
                    "tags" : [
                      "jmx",
                      "Transaction"
                    ]
                  },
                  "sort" : [
                    1617193670731,
                    2947256488
                  ]
                },
...