Fix Metric.send() to play nice with multiple metrics

[kuzmich]

In short, method signature and usage (http://docs.datadoghq.com/api/#metrics) is different and misguiding. Also, it will send the wrong data if user pass list of metrics with points as a single value. Below is a detailed explanation.

Here's a Metric.send() method without docstrings:

    @classmethod
    def send(cls, *metrics, **single_metric):
        try:
            if metrics:
                for metric in metrics:
                    if isinstance(metric, dict):
                        metric['points'] = cls._process_points(metric['points'])
                metrics_dict = {"series": metrics[0]}
            else:
                single_metric['points'] = cls._process_points(single_metric['points'])
                metrics = [single_metric]
                metrics_dict = {"series": metrics}

        except KeyError:
            raise KeyError("'points' parameter is required")

        return super(Metric, cls).send(attach_host_name=True, **metrics_dict)

There're few issues with it:

1. Method signature says it accepts arbitrary number of metrics, but api docs says it should be used with a single list, not multiple positional args. It really misguiding users.
2. If you pass a list of metrics as a single positional argument, metrics will be a list of a sinle list ([[your metrics here]]). Let's see what happens next.
   for metric in metrics will take a list of metrics and compare it with a dict - indeed, list is not a dict, so no calls to cls._process_points(). It will absolutely not change metrics in any way. Since points must be a list of lists of timestamp and a value [[timestamp, value]], wrong data will be sent. Though datadog api will say 202 Accepted, it will not not accept any data at all.
3. We can pass metrics as arbitrary args (Metric.send(*metrics)), and it will call _process_points() and transform data in the right way. But then it will take only first metric (it will be a dict), and since series must be a list, not a dict, we will get an error. And that's what I changed - 1 line of code.

[robinske]

+1, running into this problem for integrating with Runscope API tests. Any estimate on getting this added?

[remh]

@yannmh can you have a look into this one please ?

[yannmh]

Thanks a lot @kuzmich for the very detailed explanations !

1 - You are right, it's a bit confusing. While the documentation attests that the method can be used only with a single list, the examples show the same method being used with a single list and positional arguments.
I'll update the documentation to reflect the actual method state .

2 & 3 - I rebased your work, and added an extra commit on top of it to comply with the syntax demoed in the documentation examples (http://docs.datadoghq.com/api/#metrics), i.e. pass a list of metrics as a single positional argument.

Metric.send(metrics)

instead of

Metric.send(*metrics)

I opened a new PR here: #59. Thanks again

@robinske we can release it this week.

[robinske]

thanks @yannmh, ended up writing the integration without the sdk

[yannmh]

Merged with #59. Thanks again @kuzmich.