Query Frontend: fix downsampled middleware returning duplicate samples

[krya-kryak]

Downsampled middleware incorrectly determined minimum response timestamp
using only one data point, which sometimes resulted in superflous requests
and duplicate samples in response. Those duplicate samples in turn broke
graph rendering in grafana in Thanos UI.

Signed-off-by: Vladimir Kononov krya-kryak@users.noreply.github.com

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

Verification

Unit tests added for minResponseTime

[krya-kryak]

I wonder if this calls for 0.19.1.

[yeya24]

LGTM. CC @bwplotka

[krya-kryak]

For the record: this is how broken graphs looked like in thanos UI:

The response contained duplicate points for label status="timeout" up to and including 1617264240:

        "result": [
            {
                "metric": {
                    "status": "error"
                },
                "values": [
                    [1617264270, "0.05340622092250431"],
                    [1617264300, "0.042536326980866526"]
                ]
            },
            {
                "metric": {
                    "status": "timeout"
                },
                "values": [
                    [1617264180, "0.31111111111111106"],
                    [1617264210, "0.37779284253030704"],
                    [1617264240, "0.38888888888888884"],
                    [1617264180, "0.31111111111111106"],
                    [1617264210, "0.37779284253030704"],
                    [1617264240, "0.38888888888888884"],
                    [1617264270, "0.811111111111111"],
                    [1617264300, "0.6666522236665222"]
                ]
            }
        ]
    }
}

[bwplotka]

Thank you! Solid work, LGTM 🤗