[receiver/fluentforward] Fix memory leak (#32363)

**Description:** 
This fixes a memory leak happening in the Fluent Forward receiver.
`time.Sleep` was being called for a duration of 10 seconds, with no way
to interrupt during shutdown. This updates the functionality to use a
timer that can be stopped in the case of the context being cancelled.

This also enables `goleak` checks on the Fluent Forward receiver to help
ensure no goroutines are being leaked, and adds a missing `Shutdown`
call.

**Link to tracking Issue:** #30438

**Testing:** All existing tests are passing as well as added `goleak` checks.

.chloggen/goleak_fluentforwardrec.yaml

@@ -0,0 +1,27 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: bug_fix
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: fluentforwardreceiver
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Fix memory leak
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [32363]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext:
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: []

receiver/fluentforwardreceiver/package_test.go

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

receiver/fluentforwardreceiver/receiver_test.go

@@ -338,6 +338,7 @@ func TestUnixEndpoint(t *testing.T) {
 	receiver, err := newFluentReceiver(receivertest.NewNopCreateSettings(), conf, next)
 	require.NoError(t, err)
 	require.NoError(t, receiver.Start(ctx, nil))
+	defer func() { require.NoError(t, receiver.Shutdown(ctx)) }()
 
 	conn, err := net.Dial("unix", receiver.(*fluentReceiver).listener.Addr().String())
 	require.NoError(t, err)

receiver/fluentforwardreceiver/server.go

@@ -54,8 +54,14 @@ func (s *server) handleConnections(ctx context.Context, listener net.Listener) {
 		// keep trying to accept connections if at all possible. Put in a sleep
 		// to prevent hot loops in case the error persists.
 		if err != nil {
-			time.Sleep(10 * time.Second)
-			continue
+			timer := time.NewTimer(10 * time.Second)
+			select {
+			case <-ctx.Done():
+				timer.Stop()
+				return
+			case <-timer.C:
+				continue
+			}
 		}
 		stats.Record(ctx, observ.ConnectionsOpened.M(1))