Fix param_query omitted in query frontend query stats log

[yeya24]

What this PR does:

We found that param_query unabled to be logged in query frontend in latest Cortex version caused by #5622.

The bug is that, queryFields is of type []string, fields is of type []interface{}.

fields = append(fields, queryFields)

This append will add queryFields []string as a single element to fields slice, causing the params unable to be logged.
The correct way is to add key, value as two separate fields using fields = append(fields, queryFields...).

func formatQueryString(queryString url.Values) (fields []interface{}) {
	var queryFields []string
	for k, v := range queryString {
		// If `query` or `match[]` field exists, we always put it as the last field.
		if k == "query" || k == "match[]" {
			queryFields = []string{fmt.Sprintf("param_%s", k), strings.Join(v, ",")}
			continue
		}
		fields = append(fields, fmt.Sprintf("param_%s", k), strings.Join(v, ","))
	}
	if len(queryFields) > 0 {
		fields = append(fields, queryFields)
	}
	return fields
}

A unit test case has been added to ensure the correctness.

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[yeya24]

This change will also be cherry-picked into the release branch