Use a map to describe discovered labels, as they are not validated by the server

[okushchenko]

The /targets endpoint generates a JSON decoding error when you try to send a request to it while having the Node Exporter configured in Prometheus. It looks like discovered labels are being presented as is, without any validation. In particular I received this error: "__param_collect[]" is not a valid label name.

Here is the part of Prometheus server code that uses a map[string]string to describe discovered labels internally.
https://github.com/prometheus/prometheus/blob/a1f34bec2e6584a2fee9aec901f3157e3e12cbaa/web/api/v1/api.go#L505-L517

[beorn7]

@krasi-georgiev this one is for you.

[krasi-georgiev]

@okushchenko thanks the change looks valid.
can you provide some code how are you using it as I am a bit confused that you mention the /rules endpoint but the PR adds changes for the ActiveTargets

[okushchenko]

@krasi-georgiev sorry, that was just a typo. I'm using both of these endpoints, but only one of them (/targets) fails. The code example (parameters initialization was omitted for simplicity):

import (
	promapiv1 "github.com/prometheus/client_golang/api/prometheus/v1"
)

promAPI := promapiv1.NewAPI(client)
err = promAPI.Targets(ctx)
if err != nil {
	return err
}

[krasi-georgiev]

at my end I don't get any errors and get the targets as expected when I run:

func TestTargets(t *testing.T) {
	client, err := api.NewClient(api.Config{
		Address: "http://127.0.0.1:9090",
	})
	if err != nil {
		t.Fatal("error creating HTTP client", err)
	}
	promAPI := v1.NewAPI(client)
	res, err := promAPI.Targets(context.Background())
	if err != nil {
		t.Fatal("getting targets", err)
	}
	fmt.Println(res)
}

Prometheus is running at http://127.0.0.1:9090

[okushchenko]

@krasi-georgiev in order to reproduce this issue you can use the node_exporter registered as a target in Prometheus and use a list parameters to filter collectors to recreate this issue: https://github.com/prometheus/node_exporter#filtering-enabled-collectors
These scrape parameters are then being used by Prometheus as URL encoded parameters to scrape Node Exporter. These params are also apparently being present in DiscoveredLabels causing the error that I mentioned initially.

For example, when I try to query the /api/v1/targets endpoint in Prometheus the response contains a target that has this map of discovered labels:

"discoveredLabels": {
    "__address__": "localhost:19100",
    "__metrics_path__": "/metrics",
    "__param_collect[]": "arp",
    "__scheme__": "http",
    "host": "localhost",
    "job": "node",
}

[krasi-georgiev]

yep I was able to reproduce and your PR fixes the issue.

We started a discussion to see if we can improve these tests by using the actual Prom api module and some sample data instead of mocking the responses.
#528