Replace aggregation key with formatted values

[jpountz]

Many aggregations provide two representations of aggregated values, such as key and key_as_string in terms/histogram aggregations. This is annoying because this is duplicate information and the key return value leaks the internal representation of data in doc values. For instance this forced us to break the response of terms or range aggregations on ip fields when we added ipv6 support since we do not store ip addresses in a number anymore. We should return a single key parameter and drop key_as_string.

[cbuescher]

As a note, key_as_string however is currently used for formatting utc keys for date histograms according to the time zone specified for rounding in the query. We introduced this back in #9710, if we remove this every bucket key needs to be converted to the appropriate time zone on the user side to display the correct local time on the buckets.

[colings86]

@jpountz is this still something you think we should do?

/cc @elastic/es-search-aggs

[nik9000]

@jpountz reviving this, maybe, do you think we should just return the string valued keys? Not the "native" keys?

[jpountz]

@nik9000 Yes, we shouldn't leak the internal representation of the data to our users. It makes the API look ugly and makes changing the way our field types are implemented breaking. Ideally we would make keys go through MappedFieldType#docValuesFormat like we do with docvalue_fields?

[nik9000]

Got it.

[not-napoleon]

Related (possibly a dup of) #63898

[mitar]

I thin it would be better if one could choose between internal representation, key_as_string, or some other formatting. For us, we prefer internal representation and as little of changes to the key as possible. (We use then key values to construct filters and so we can pass values as-is.)

[wchaparro]

closing as not planned.