[chore][exporter/alertmanagerexporter] Enable goleak check (#30710)

**Description:** <Describe what has changed.>
<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
Enables goleak check on tests in the `alertmanager` exporter to ensure
it's not leaking goroutines. This is a test only change.

**Link to tracking Issue:** <Issue number if applicable>
#30438

**Testing:** <Describe what testing was performed and which tests were
added.>
All tests are passing, goleak check is passing.

Author: crobert-1

exporter/alertmanagerexporter/alertmanager_exporter_test.go

@@ -308,6 +308,7 @@ func newMockServer(t *testing.T) *MockServer {
 
 func TestAlertManagerPostAlert(t *testing.T) {
 	mock := newMockServer(t)
+	defer func() { mock.mockserver.Close() }()
 	factory := NewFactory()
 	cfg := factory.CreateDefaultConfig().(*Config)
 

exporter/alertmanagerexporter/go.mod

@@ -20,6 +20,7 @@ require (
 	go.opentelemetry.io/collector/semconv v0.93.1-0.20240124123350-9047c0e373f9
 	go.opentelemetry.io/otel/metric v1.22.0
 	go.opentelemetry.io/otel/trace v1.22.0
+	go.uber.org/goleak v1.3.0
 	go.uber.org/zap v1.26.0
 )
 

exporter/alertmanagerexporter/go.sum

@@ -0,0 +1,17 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package alertmanagerexporter
+
+import (
+	"testing"
+
+	"go.uber.org/goleak"
+)
+
+// The IgnoreTopFunction call prevents catching the leak generated by opencensus
+// defaultWorker.Start which at this time is part of the package's init call.
+// See https://github.com/census-instrumentation/opencensus-go/issues/1191 for more information.
+func TestMain(m *testing.M) {
+	goleak.VerifyTestMain(m, goleak.IgnoreTopFunction("go.opencensus.io/stats/view.(*worker).start"))
+}