ref(AttributeValues): add arrays to response

[MeredithAnya]

Changes the TraceItemAttributeValues response so that instead of returning two
parallel lists (values and counts) that callers have to zip together by index,
we return a single list of ValueData objects. Each one carries everything we know
about a single attribute value:

message ValueData {
  AttributeValue value = 1;
  int64 count = 2;
  google.protobuf.Timestamp last_seen = 3;
}

Why:

• Non-string values. value is an AttributeValue rather than a string, so
  the endpoint can return values for non-string attributes, including arrays
  (e.g. the attribute_array_string column) via AttributeValue.val_array —
  this was the original motivation for this PR, see the draft Snuba
  implementation in ref(attrvalues): support array-of-string attributes and typed value_data snuba#8390.
• Room to grow. Adding per-value metadata is now a new field on ValueData
  instead of another parallel list. last_seen is the first of those.

values and counts are marked deprecated rather than removed, so existing
callers keep working until they migrate to value_data.

[linear-code]

EAP-698

[github-actions]

The latest Buf updates on your PR. Results from workflow ci / buf-checks (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Aug 25, 2026, 10:28 PM

[phacops]

I think it would be nice to plan to return a count and a last seen for each value we see.

So maybe, we switch to having a repeated type that countains a oneof of scalar (string, int, double, bool) and a count and last_seen field as well.

[MeredithAnya]

I think it would be nice to plan to return a count and a last seen for each value we see.

So maybe, we switch to having a repeated type that countains a oneof of scalar (string, int, double, bool) and a count and last_seen field as well.

This isn't hitting the eap_item_co_occurring_attrs table so there isn't the last_seen field. Also right now the endpoint accepts on strings or bool scalar types and it converts the bool type to string response, so was going to have the response be consistent there for arrays