Replica labels shouldn't be added to exemplar labels

[yeya24]

Thanos, Prometheus and Golang version used:

Latest

What happened:

While working on #4221, I found for exemplar APIs, replica labels are added to both series labels and exemplar labels.

For example, one example response is:

curl 'localhost:49463/api/v1/query_exemplars?end=1620780122.472&query=http_request_duration_seconds_bucket%7Bhandler%3D%22label_names%22%7D&start=1620772922.472' | jq .

{
  "status": "success",
  "data": [
    {
      "seriesLabels": {
        "__name__": "http_request_duration_seconds_bucket",
        "code": "200",
        "handler": "label_names",
        "instance": "e2e_test_exemplars_fanout-querier-query:8080",
        "job": "myself",
        "le": "0.01",
        "method": "get",
        "prometheus": "ha"
      },
      "exemplars": [
        {
          "labels": {
            "prometheus": "ha",
            "traceID": "69ab3ca4a0b7f639"
          },
          "timestamp": 1620776522.474,
          "value": "0.001569066"
        }
      ]
    }
  ]
}

Here prometheus=ha is the external label. It should be added to the series labels, but not to exemplar labels.

What you expected to happen:
prometheus=ha appears in series labels and doesn't appear in exemplar labels.