[chore][exporter/prometheusremotewriteexporter] Enable goleak check (#30966)

**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` checks for the prometheusremotewrite exporter, to
help ensure no goroutines are being leaked. This is a test only change,
the only update is to add a missing `Shutdown` call.

**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 added `goleak` check.

---------

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

Author: crobert-1

exporter/prometheusremotewriteexporter/factory_test.go

@@ -86,6 +86,7 @@ func Test_createMetricsExporter(t *testing.T) {
 				return
 			}
 			assert.NoError(t, err)
+			assert.NoError(t, exp.Shutdown(context.Background()))
 		})
 	}
 }

exporter/prometheusremotewriteexporter/go.mod

@@ -25,6 +25,7 @@ require (
 	go.opentelemetry.io/collector/pdata v1.2.0
 	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/multierr v1.11.0
 	go.uber.org/zap v1.27.0
 )

exporter/prometheusremotewriteexporter/package_test.go

@@ -0,0 +1,17 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+package prometheusremotewriteexporter
+
+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)
+}