IntermediateTermBucketResult on numeric types is not serializable to json

[PSeitz]

Number keys are not supported in JSON.
Serialization fails for FxHashMap<Key::F64(_), _>

#[derive(Default, Clone, Debug, PartialEq, Serialize, Deserialize)]
/// Term aggregation including error counts
pub struct IntermediateTermBucketResult {
    pub(crate) entries: FxHashMap<Key, IntermediateTermBucketEntry>,
    pub(crate) sum_other_doc_count: u64,
    pub(crate) doc_count_error_upper_bound: u64,
}

#[derive(Clone, Debug, Serialize, Deserialize, PartialOrd)]
/// The key to identify a bucket.
#[serde(untagged)]
pub enum Key {
    /// String key
    Str(String),
    /// `f64` key
    F64(f64),
}