[chore][extension/opampextension] Enable goleak check (#31175)

**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.-->
This enables the `goleak` check for the opamp extension, to help ensure
it's not leaking any goroutines. This is a test only change, a
`Shutdown` call was missing.

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

**Testing:** <Describe what testing was performed and which tests were
added.>
All existing tests are passing, as well as the added `goleak` check

---------

Co-authored-by: Antoine Toulme <[email protected]>
Co-authored-by: Pablo Baeyens <[email protected]>

Author: 3 people

extension/opampextension/go.mod

@@ -16,6 +16,7 @@ require (
 	go.opentelemetry.io/collector/semconv v0.95.0
 	go.opentelemetry.io/otel/metric v1.23.1
 	go.opentelemetry.io/otel/trace v1.23.1
+	go.uber.org/goleak v1.3.0
 	go.uber.org/zap v1.27.0
 	gopkg.in/yaml.v3 v3.0.1
 )

extension/opampextension/opamp_agent_test.go

@@ -113,4 +113,5 @@ func TestStart(t *testing.T) {
 	assert.NoError(t, err)
 
 	assert.NoError(t, o.Start(context.TODO(), componenttest.NewNopHost()))
+	assert.NoError(t, o.Shutdown(context.TODO()))
 }

extension/opampextension/package_test.go

@@ -0,0 +1,14 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package opampextension
+
+import (
+	"testing"
+
+	"go.uber.org/goleak"
+)
+
+func TestMain(m *testing.M) {
+	goleak.VerifyTestMain(m)
+}