[Filebeat] Reduce memory usage of multiline (#14068) (#14073)

The use of time.After when multiline is enabled and lines don't match
the multiline pattern increases the memory usage (from 30MB to 1GB).

This extra memory is attributed to unexpired timers allocated internally
by the Go runtime when time.After(duration) is used. According to the
docs: "If efficiency is a concern, use NewTimer instead and call Timer.Stop
if the timer is no longer needed.".

It's not clear to me why this problem only appears when many lines on
the input file don't match the pattern.

(cherry picked from commit ce651e0)

CHANGELOG.next.asciidoc

@@ -47,6 +47,7 @@ https://github.com/elastic/beats/compare/v7.0.0-alpha2...master[Check the HEAD d
 
 *Filebeat*
 
+- Fixed increased memory usage with large files when multiline pattern does not match. {issue}14068[14068]
 
 *Heartbeat*
 

libbeat/reader/readfile/timeout.go

@@ -28,7 +28,7 @@ var (
 	errTimeout = errors.New("timeout")
 )
 
-// timeoutProcessor will signal some configurable timeout error if no
+// TimeoutReader will signal some configurable timeout error if no
 // new line can be returned in time.
 type TimeoutReader struct {
 	reader  reader.Reader
@@ -43,7 +43,7 @@ type lineMessage struct {
 	err  error
 }
 
-// New returns a new timeout reader from an input line reader.
+// NewTimeoutReader returns a new timeout reader from an input line reader.
 func NewTimeoutReader(reader reader.Reader, signal error, t time.Duration) *TimeoutReader {
 	if signal == nil {
 		signal = errTimeout
@@ -75,14 +75,15 @@ func (r *TimeoutReader) Next() (reader.Message, error) {
 			}
 		}()
 	}
-
+	timer := time.NewTimer(r.timeout)
 	select {
 	case msg := <-r.ch:
 		if msg.err != nil {
 			r.running = false
 		}
+		timer.Stop()
 		return msg.line, msg.err
-	case <-time.After(r.timeout):
+	case <-timer.C:
 		return reader.Message{}, r.signal
 	}
 }