Invalid summary/histogram labels when the metric has an empty label set ({})

[mkuratczyk]

If a summary or histogram is registered with an empty, but present, label set, it is returned with an incorrect
label set because the quantile/vmrange labels are appended with a comma, but there are no labels
before the comma.

I ran into this because I allow user-set custom labels, which may be empty. If I use promutils.NewLabelsFromString or NewLabelsFromMap, and just appended the result to the metric name, I will hit this issue if the user didn't provide custom labels.
Of course I can check if the string/map is empty, or remove the {} in the example below, but it feels like this should be handled.
If a metric name like below (test_summary{}) is considered invalid, it should be rejected during registration, since there's already validation happening at that time.

Here's an executable way to reproduce the issue:

1. Register a summary/histogram with an empty label set:

package main

import (
	"net/http"

	vm "github.com/VictoriaMetrics/metrics"
)

func main() {
	summary := vm.NewSummary(`test_summary{}`)
	summary.Update(1)

	histogram := vm.NewHistogram(`test_histogram{}`)
	histogram.Update(1)

	http.HandleFunc("/metrics", func(w http.ResponseWriter, req *http.Request) {
		vm.WritePrometheus(w, false)
	})
	http.ListenAndServe("127.0.0.1:8080", nil)

}

2. Scrape the endpoint:

$ curl -s localhost:8080/metrics
test_histogram_bucket{,vmrange="8.799e-01...1.000e+00"} 1
test_histogram_sum{} 1
test_histogram_count{} 1
test_summary{,quantile="0.5"} 1
test_summary{,quantile="0.9"} 1
test_summary{,quantile="0.97"} 1
test_summary{,quantile="0.99"} 1
test_summary{,quantile="1"} 1
test_summary_sum{} 1
test_summary_count{} 1

3. test_histogram_bucket a test_summary have incorrect label sets starting with a comma.
4. If you scrape this with Prometheus, you'll get an error:

expected label name, got ",v" ("COMMA") while parsing: "test_histogram_bucket{,v"