Bad error message when passing invalid JSON to Aggregation

[PSeitz]

Error messages are like:
"Invalid aggregation request: data did not match any variant of untagged enum
Aggregation at line 18 column 2"

Currently the top level entry for an aggregation request uses #[serde(untagged)], which is trying to deserialize variants until one works. That's why we always get the above error message.

#[derive(Clone, Debug, PartialEq, Serialize, Deserialize)]
#[serde(untagged)]
pub enum Aggregation {
    /// Bucket aggregation, see [BucketAggregation] for details.
    Bucket(BucketAggregation),
    /// Metric aggregation, see [MetricAggregation] for details.
    Metric(MetricAggregation),
}

Effectively Aggregations are externally tagged (https://serde.rs/enum-representations.html).

{"terms": {...}}
{"histogram": {...}}

So ideally we would just flatten the buckets and metrics into a single enum, but serde does not support flattening on enums (serde-rs/serde#1402).

I see 3 Options currently:

• Wait for/Implement flatten on enums in serde
• Manually flatten the enums to the top-level (Internally that would be just an intermediate struct)
• Use a custom deserializer (Ideally it would only manually implement the matching on the keys ("terms", "histogram", etc.) and use the defaults for the rest)