Document that the summary export threshold is unintuitive

[mstoykov]

When the summary export was written no one noticed that the boolean used to say whether a threshold has passed or not is true when it failed and false when it has passed, which confusing to users.

I think this should be documented better and we should have an example of a threshold

links:
original PR
community forum link
k6 issue

[na--]

This is somewhat connected to grafana/k6#1768 - that PR will preserve the current --summary-export format as it is, for backwards compatibility, but the data format we will pass to handleSummary() will be saner. As an example:

{
    "metrics": {
        "http_req_duration": {
            "type": "trend",
            "contains": "time",
            "values": {
                "p(80)": 277.99444800000003,
                "p(99.99)": 408.53503457669984,
                "avg": 202.9503869583333,
                "min": 132.286877,
                "med": 152.79688099999998,
                "p(70)": 271.7887037,
                "count": 120,
                "max": 408.674962,
                "p(90)": 280.83382650000004,
                "p(95)": 339.6004894,
                "p(99)": 395.77117560000005
            },
            "thresholds": {
                "p(95)<500": {
                    "ok": true
                }
            }
        },
    }
}