[Heartbeat] Report first synth error on summary (#24194) (#24222)

Fixes elastic/kibana#92066

Prior to this for the summary.error field for browser events we'd
report the last encountered error. We now report the first. This
prevents the issue where on a failing run the synthetics library
considers an exit status of 1 to be an error. This will always be the
case when a step fails, meaning that the last error will always be a
status code error stepping on the actual step error.

(cherry picked from commit c5821d9)

x-pack/heartbeat/monitors/browser/synthexec/enrich.go

@@ -36,7 +36,7 @@ type journeyEnricher struct {
 	journey         *Journey
 	checkGroup      string
 	errorCount      int
-	lastError       error
+	firstError      error
 	stepCount       int
 	// The first URL we visit is the URL for this journey, which is set on the summary event.
 	// We store the URL fields here for use on the summary event.
@@ -69,7 +69,7 @@ func (je *journeyEnricher) enrich(event *beat.Event, se *SynthEvent) error {
 		// Record start and end so we can calculate journey duration accurately later
 		switch se.Type {
 		case "journey/start":
-			je.lastError = nil
+			je.firstError = nil
 			je.checkGroup = makeUuid()
 			je.journey = se.Journey
 			je.start = event.Timestamp
@@ -121,7 +121,9 @@ func (je *journeyEnricher) enrichSynthEvent(event *beat.Event, se *SynthEvent) e
 	if se.Error != nil {
 		jobErr = stepError(se.Error)
 		je.errorCount++
-		je.lastError = jobErr
+		if je.firstError == nil {
+			je.firstError = jobErr
+		}
 	}
 
 	return jobErr
@@ -154,7 +156,7 @@ func (je *journeyEnricher) createSummary(event *beat.Event) error {
 				"down": down,
 			},
 		})
-		return je.lastError
+		return je.firstError
 	}
 
 	return fmt.Errorf("journey did not finish executing, %d steps ran", je.stepCount)

x-pack/heartbeat/monitors/browser/synthexec/enrich_test.go

@@ -30,6 +30,11 @@ func TestJourneyEnricher(t *testing.T) {
 		Name:    "my-errname",
 		Stack:   "my\nerr\nstack",
 	}
+	otherErr := &SynthError{
+		Message: "last-errmsg",
+		Name:    "last-errname",
+		Stack:   "last\nerr\nstack",
+	}
 	journeyStart := &SynthEvent{
 		Type:                 "journey/start",
 		TimestampEpochMicros: 1000,
@@ -66,7 +71,7 @@ func TestJourneyEnricher(t *testing.T) {
 		makeStepEvent("step/start", 21, "Step2", 1, "", nil),
 		makeStepEvent("step/end", 30, "Step2", 1, url2, syntherr),
 		makeStepEvent("step/start", 31, "Step3", 1, "", nil),
-		makeStepEvent("step/end", 40, "Step3", 1, url3, nil),
+		makeStepEvent("step/end", 40, "Step3", 1, url3, otherErr),
 		journeyEnd,
 	}