metrics: Search API should return list of traces when present for datapoints

The metrics search API response should contain a list of trace ids per data point, e.g.

{
    "start_ts": 1724178357456859869,
    "end_ts": 1724180157456859869,
    "results": [
        {
            "name": "exporter_send_failed_log_records",
            "description": "Number of log records in failed attempts to send to destination.",
            "unit": "1",
            "type": "Sum",
            "attributes": {
                "exporter": "gitlabobservability",
                "gob.cluster_dns_domain": "observe.gitlab.com",
                "gob.cluster_name": "opstrace-prod",
                "gob.cluster_target": "gcp"
            },
            "values": [
                [
                    "1724178357000000000",
                    "0.000000",
                    [traceId_1, traceId_2, ...] <- empty means no exemplars connected to this datapoint
                ],
                ....

TBD: What about histograms? ( opstrace#146 (comment 2069413154))