Feature(new_metrics): support filters for selected metric fields in response

[empiredan]

1. Motivation

According to #1206, with_metric_fields and without_metric_fields will be provided as the parameters in the RESTful API to filter the fields of metric in the response to client.

2. Usage for with_metric_fields

with_metric_fields will ask for the metric fields that are not included in the request. For example, if the request is given as below:

/metrics?with_metric_fields=value,p99

The response will only contain the fields of "value" and "p99":

[
    { // entity 1
        "metrics": [
            // gauge
            {
                "value": 100
            },
            // counter
            {
                "value": 10000
            },
            // percentile
            {
                "p99": 10
            }
        ]
    },
    ...
]

3. Usage for without_metric_fields

Similarly, without_metric_fields will ask for the metric fields that are not included in the request. For example, if the request is given as below:

/metrics?without_metric_fields=value,p99

The response will contain all the fields except "value" and "p99":

[
    { // entity 1
        "metrics": [
            // gauge
            {
                "name": "<gauge_name>"
            },
            // counter
            {
                "name": "<counter_name>"
            },
            // percentile
            {
                "name": "<percentile_name>",
                "p50": 1,
                "p90": 5,
                "p95": 8,
                "p999": 15
            }
        ]
    },
    ...
]

4. Wrong usage

Once both with_metric_fields and without_metric_fields are provided in the request, it will considered as invalid. We can discuss the reason in 2 conditions.

Firstly, suppose both with_metric_fields and without_metric_fields are provided, and they include the same fields. In this condition, each conflicts with another. For example, with_metric_fields=a,b and without_metric_fields=b,c will lead to contradiction: we cannot decide if b should be put in the response to client.

On the contrary, suppose with_metric_fields and without_metric_fields do not include any same field. In this condition, they can be simplified as a unique with_metric_fields. For example, with_metric_fields=a,b and without_metric_fields=c,d can be simplified as with_metric_fields=a,b: without_metric_fields=c,d is useless here since we have declared that only a and b fields are needed according to with_metric_fields=a,b.

Therefore, we can draw a conclusion that with_metric_fields and without_metric_fields should not be provided at the same time.