[chore][exporter/prometheusexporter] Enable goleak check (#30965)

**Description:** 
This enables the `goleak` package for testing the Prometheus exporter.
This helps to ensure no goroutines are leaked by this component. This is
a test only change, the only updates are making sure all tests close the
response body of `Get` calls.

**Link to tracking Issue:**
#30438

**Testing:** 
All tests are passing, including added `goleak` test.

exporter/prometheusexporter/go.mod

@@ -22,6 +22,7 @@ require (
 	go.opentelemetry.io/collector/semconv v0.96.1-0.20240322165517-15201f1e5967
 	go.opentelemetry.io/otel/metric v1.24.0
 	go.opentelemetry.io/otel/trace v1.24.0
+	go.uber.org/goleak v1.3.0
 	go.uber.org/zap v1.27.0
 	gopkg.in/yaml.v2 v2.4.0
 )

exporter/prometheusexporter/package_test.go

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

exporter/prometheusexporter/prometheus_test.go

@@ -136,8 +136,10 @@ func TestPrometheusExporter_WithTLS(t *testing.T) {
 	t.Cleanup(func() {
 		require.NoError(t, exp.Shutdown(context.Background()))
 		// trigger a get so that the server cleans up our keepalive socket
-		_, err = httpClient.Get("https://localhost:7777/metrics")
+		var resp *http.Response
+		resp, err = httpClient.Get("https://localhost:7777/metrics")
 		require.NoError(t, err)
+		require.NoError(t, resp.Body.Close())
 	})
 
 	assert.NotNil(t, exp)
@@ -195,8 +197,10 @@ func TestPrometheusExporter_endToEndMultipleTargets(t *testing.T) {
 	t.Cleanup(func() {
 		require.NoError(t, exp.Shutdown(context.Background()))
 		// trigger a get so that the server cleans up our keepalive socket
-		_, err = http.Get("http://localhost:7777/metrics")
+		var resp *http.Response
+		resp, err = http.Get("http://localhost:7777/metrics")
 		require.NoError(t, err)
+		require.NoError(t, resp.Body.Close())
 	})
 
 	assert.NotNil(t, exp)
@@ -277,8 +281,10 @@ func TestPrometheusExporter_endToEnd(t *testing.T) {
 	t.Cleanup(func() {
 		require.NoError(t, exp.Shutdown(context.Background()))
 		// trigger a get so that the server cleans up our keepalive socket
-		_, err = http.Get("http://localhost:7777/metrics")
+		var resp *http.Response
+		resp, err = http.Get("http://localhost:7777/metrics")
 		require.NoError(t, err)
+		require.NoError(t, resp.Body.Close())
 	})
 
 	assert.NotNil(t, exp)
@@ -354,8 +360,10 @@ func TestPrometheusExporter_endToEndWithTimestamps(t *testing.T) {
 	t.Cleanup(func() {
 		require.NoError(t, exp.Shutdown(context.Background()))
 		// trigger a get so that the server cleans up our keepalive socket
-		_, err = http.Get("http://localhost:7777/metrics")
+		var resp *http.Response
+		resp, err = http.Get("http://localhost:7777/metrics")
 		require.NoError(t, err)
+		require.NoError(t, resp.Body.Close())
 	})
 
 	assert.NotNil(t, exp)
@@ -434,8 +442,10 @@ func TestPrometheusExporter_endToEndWithResource(t *testing.T) {
 	t.Cleanup(func() {
 		require.NoError(t, exp.Shutdown(context.Background()))
 		// trigger a get so that the server cleans up our keepalive socket
-		_, err = http.Get("http://localhost:7777/metrics")
+		var resp *http.Response
+		resp, err = http.Get("http://localhost:7777/metrics")
 		require.NoError(t, err, "Failed to perform a scrape")
+		require.NoError(t, resp.Body.Close())
 	})
 
 	assert.NotNil(t, exp)