Inconsistent results when using logfmt parser stage with fields

[chaudum]

Queries that aggregate on extracted labels (sum by (label_a, label_b)) return different results when using | logfmt and | logfmt label_a, label_b.

Example queries:

$ logcli instant-query 'sum by (err, msg) (count_over_time({**********} |= "level=error" | logfmt [24h]))'  | jq . 

[
  {
    "metric": {
      "err": "**********",
      "msg": "failed to do request"
    },
    "value": [
      1732717258.183,
      "2"
    ]
  },
  {
    "metric": {
      "err": "**********",
      "msg": "backoff terminated unsuccessfully"
    },
    "value": [
      1732717258.183,
      "1"
    ]
  },
  {
    "metric": {
      "msg": "Error in comparison client"
    },
    "value": [
      1732717258.183,
      "1"
    ]
  }
]

vs

$ logcli instant-query 'sum by (err, msg) (count_over_time({**********} |= "level=error" | logfmt err, msg [24h]))'  | jq .

[
  {
    "metric": {
      "msg": "Error in comparison client"
    },
    "value": [
      1732717323.361,
      "1"
    ]
  },
  {
    "metric": {
      "msg": "backoff terminated unsuccessfully"
    },
    "value": [
      1732717323.361,
      "1"
    ]
  },
  {
    "metric": {
      "msg": "failed to do request"
    },
    "value": [
      1732717323.361,
      "2"
    ]
  }
]

The second query, where err and msg are part of the logfmt parser stage, is missing the err label in the series of the result.

When adding another label, which does not even need to exist, to the aggregation, the err label becomes part of the series again:

$ logcli instant-query 'sum by (err, msg, foo) (count_over_time({**********} |= "level=error" | logfmt err,msg [24h]))'  | jq .

[
  {
    "metric": {
      "err": "**********",
      "msg": "failed to do request"
    },
    "value": [
      1732717574.53,
      "2"
    ]
  },
  {
    "metric": {
      "err": "**********",
      "msg": "backoff terminated unsuccessfully"
    },
    "value": [
      1732717574.53,
      "1"
    ]
  },
  {
    "metric": {
      "msg": "Error in comparison client"
    },
    "value": [
      1732717574.53,
      "1"
    ]
  }
]