Fix type issue for threshold in screenboard rule

[anatolebeuzon]

Because of a type issue, we could not submit thresholds such as 99.9 for rules in the uptime widget.

This PR fixes that.

[nyanshak]

I'm having trouble finding any documentation about an "uptime" screen widget, so any testing I've been doing to validate this has been based on the code in the integration tests. Do you have any doc links you could share to make this a bit easier?

[anatolebeuzon]

@nyanshak Actually the uptime widget is purposefully undocumented and still a WIP at this point. Sorry about that, missed it in the first PR. I added a warning in the code telling people not to rely too much on it.

[nyanshak]

The reason for using json.Number, at least across many of Datadog's API endpoints, is that it's common for the API to accept (and send) multiple types.

I just tested against the screenboard API to make a screenboard with uptime widget with thresholds:

    {
      ... truncated ...
      "rules": {
        "1": {
          "threshold": "98", // Hmm.. this can be a string
          "color": "red",
          "timeframe": "7 days"
        },
        "0": {
          "threshold": "95.1", // Hmm, this can also be a string
          "color": "green",
          "timeframe": "Month-to-date"
        }
      },
      "type": "uptime"
    }

I'm able to send thresholds as strings (and Datadog subsequently sends them back as strings). Using float64 as the type results in errors somewhat like this:

json: cannot unmarshal string into Go struct field Rule.Threshold of type float64

Looking back, I believe this is originally the reason we started using json.Number rather than float64, as this case is handled properly there.

Besides that issue with the type, I'm happy to merge this.