Return an error if metric cannot be registered

`prometheus.MustRegister` panics if the metric cannot be registered.
Instead, use `prometheus.Register` and return the error if the metric
cannot be registered.

Also, add tests for `RegisterMetricAndTrackRateLimiterUsage`.

Signed-off-by: Ferran Rodenas <frodenas@gmail.com>

pkg/util/metrics/BUILD

@@ -3,6 +3,7 @@ package(default_visibility = ["//visibility:public"])
 load(
     "@io_bazel_rules_go//go:def.bzl",
     "go_library",
+    "go_test",
 )
 
 go_library(
@@ -16,6 +17,15 @@ go_library(
     ],
 )
 
+go_test(
+    name = "go_default_test",
+    srcs = ["util_test.go"],
+    library = ":go_default_library",
+    deps = [
+        "//vendor/k8s.io/client-go/util/flowcontrol:go_default_library",
+    ],
+)
+
 filegroup(
     name = "package-srcs",
     srcs = glob(["**"]),

pkg/util/metrics/util.go

@@ -51,7 +51,9 @@ func registerRateLimiterMetric(ownerName string) error {
 		Help:      fmt.Sprintf("A metric measuring the saturation of the rate limiter for %v", ownerName),
 	})
 	rateLimiterMetrics[ownerName] = metric
-	prometheus.MustRegister(metric)
+	if err := prometheus.Register(metric); err != nil {
+		return fmt.Errorf("error registering rate limiter usage metric: %v", err)
+	}
 	return nil
 }
 

pkg/util/metrics/util_test.go

@@ -0,0 +1,54 @@
+/*
+Copyright 2017 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package metrics
+
+import (
+	"strings"
+	"testing"
+
+	"k8s.io/client-go/util/flowcontrol"
+)
+
+func TestRegisterMetricAndTrackRateLimiterUsage(t *testing.T) {
+	testCases := []struct {
+		ownerName   string
+		rateLimiter flowcontrol.RateLimiter
+		err         string
+	}{
+		{
+			ownerName:   "owner_name",
+			rateLimiter: flowcontrol.NewTokenBucketRateLimiter(1, 1),
+			err:         "",
+		},
+		{
+			ownerName:   "invalid-owner-name",
+			rateLimiter: flowcontrol.NewTokenBucketRateLimiter(1, 1),
+			err:         "error registering rate limiter usage metric",
+		},
+	}
+
+	for i, tc := range testCases {
+		e := RegisterMetricAndTrackRateLimiterUsage(tc.ownerName, tc.rateLimiter)
+		if e != nil {
+			if tc.err == "" {
+				t.Errorf("[%d] unexpected error: %v", i, e)
+			} else if !strings.Contains(e.Error(), tc.err) {
+				t.Errorf("[%d] expected an error containing %q: %v", i, tc.err, e)
+			}
+		}
+	}
+}