[chore][exporter/fileexporter] Enable goleak check (#30899)

**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 `goleak` to check for any leaking goroutines in this
package. There is an existing goleak, but it's in a third party
dependency which we cannot resolve on our end. There's been a [PR open
for this issue](natefinch/lumberjack#57) for 6
years and it hasn't been merged.

**Note:** The `natefinch/lumberjack` package is now unmaintained, so
this will likely never be fixed. We should consider moving to a new
package if possible to avoid the leak. I'll defer to code owners for
feedback on possible options.

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

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

exporter/fileexporter/go.mod

@@ -14,6 +14,7 @@ require (
 	go.opentelemetry.io/collector/pdata v1.0.2-0.20240130181942-9c7177496fd5
 	go.opentelemetry.io/otel/metric v1.22.0
 	go.opentelemetry.io/otel/trace v1.22.0
+	go.uber.org/goleak v1.3.0
 	gopkg.in/natefinch/lumberjack.v2 v2.2.1
 )
 

exporter/fileexporter/package_test.go

@@ -0,0 +1,16 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package fileexporter
+
+import (
+	"testing"
+
+	"go.uber.org/goleak"
+)
+
+// The IgnoreTopFunction call prevents catching the leak generated by the natefinch/lumberjack package.
+// Existing issue for leak: https://github.com/natefinch/lumberjack/issues/56
+func TestMain(m *testing.M) {
+	goleak.VerifyTestMain(m, goleak.IgnoreTopFunction("gopkg.in/natefinch/lumberjack%2ev2.(*Logger).millRun"))
+}