def METHOD_NAME(self, assembly):
    """
    Update the "stats" entry inside the "_meta" field of the index's mapping. Return the updated "meta._stats" field of the mapping.
    Args:
        assembly (str): a string of "hg19" or "hg38"
    """
    """
    With ES7, self.client.indices.get_mapping(index_name) will return a dict like
        {
            "<index_name>": {
                "mappings": {
                    "_meta": {
                        "build_date": "2021-08-29T15:43:59.554260-07:00",
                        "biothing_type": "variant",
                        "stats": {
                            "total": ...
                            "vcf": ...
                            "observed": ...
                            "<assembly>": ...
                        },
                        "src": {...},
                        "build_version": "20210829"
                    },
                    "properties": {...}
                }
            }
        }
    The goal here is to update the "stats" field inside.
    """
    stats = dict()
    stats["total"] = self.client.count(index=self.index_name)["count"]
    for field in [assembly, "observed", "vcf"]:
        body = {"query": {"exists": {"field": field}}}
        stats[field] = self.client.count(index=self.index_name, body=body)["count"]
    mapping = self.client.indices.get_mapping(index=self.index_name)
    meta = mapping[self.index_name]["mappings"]["_meta"]  # Get the current meta field from mapping
    meta.get("stats", {}).update(stats)  # Update the meta content
    self.client.indices.put_mapping(body={"_meta": meta}, index=self.index_name)  # Write the modified meta to ES mapping
    return meta["stats"]