HTTP SD: Allow charset in content type (#8981)

* Added content type regex

Signed-off-by: Levi Harrison <git@leviharrison.dev>

discovery/http/http.go

@@ -21,7 +21,9 @@ import (
 	"io/ioutil"
 	"net/http"
 	"net/url"
+	"regexp"
 	"strconv"
+	"strings"
 	"time"
 
 	"github.com/go-kit/log"
@@ -41,7 +43,8 @@ var (
 		RefreshInterval:  model.Duration(60 * time.Second),
 		HTTPClientConfig: config.DefaultHTTPClientConfig,
 	}
-	userAgent = fmt.Sprintf("Prometheus/%s", version.Version)
+	userAgent        = fmt.Sprintf("Prometheus/%s", version.Version)
+	matchContentType = regexp.MustCompile(`^(?i:application\/json(;\s*charset=("utf-8"|utf-8))?)$`)
 )
 
 func init() {
@@ -152,7 +155,7 @@ func (d *Discovery) refresh(ctx context.Context) ([]*targetgroup.Group, error) {
 		return nil, errors.Errorf("server returned HTTP status %s", resp.Status)
 	}
 
-	if resp.Header.Get("Content-Type") != "application/json" {
+	if !matchContentType.MatchString(strings.TrimSpace(resp.Header.Get("Content-Type"))) {
 		return nil, errors.Errorf("unsupported content type %q", resp.Header.Get("Content-Type"))
 	}
 

discovery/http/http_test.go

@@ -104,3 +104,61 @@ func TestHTTPInvalidFormat(t *testing.T) {
 	_, err = d.refresh(ctx)
 	require.EqualError(t, err, `unsupported content type "text/plain; charset=utf-8"`)
 }
+
+func TestContentTypeRegex(t *testing.T) {
+	cases := []struct {
+		header string
+		match  bool
+	}{
+		{
+			header: "application/json;charset=utf-8",
+			match:  true,
+		},
+		{
+			header: "application/json;charset=UTF-8",
+			match:  true,
+		},
+		{
+			header: "Application/JSON;Charset=\"utf-8\"",
+			match:  true,
+		},
+		{
+			header: "application/json; charset=\"utf-8\"",
+			match:  true,
+		},
+		{
+			header: "application/json",
+			match:  true,
+		},
+		{
+			header: "application/jsonl; charset=\"utf-8\"",
+			match:  false,
+		},
+		{
+			header: "application/json;charset=UTF-9",
+			match:  false,
+		},
+		{
+			header: "application /json;charset=UTF-8",
+			match:  false,
+		},
+		{
+			header: "application/ json;charset=UTF-8",
+			match:  false,
+		},
+		{
+			header: "application/json;",
+			match:  false,
+		},
+		{
+			header: "charset=UTF-8",
+			match:  false,
+		},
+	}
+
+	for _, test := range cases {
+		t.Run(test.header, func(t *testing.T) {
+			require.Equal(t, test.match, matchContentType.MatchString(test.header))
+		})
+	}
+}