use AddError everywhere (#2372)

plugins/inputs/prometheus/prometheus.go

@@ -67,19 +67,17 @@ var ErrProtocolError = errors.New("prometheus protocol error")
 func (p *Prometheus) Gather(acc telegraf.Accumulator) error {
 	var wg sync.WaitGroup
 
-	var outerr error
-
 	for _, serv := range p.Urls {
 		wg.Add(1)
 		go func(serv string) {
 			defer wg.Done()
-			outerr = p.gatherURL(serv, acc)
+			acc.AddError(p.gatherURL(serv, acc))
 		}(serv)
 	}
 
 	wg.Wait()
 
-	return outerr
+	return nil
 }
 
 var tr = &http.Transport{

plugins/inputs/prometheus/prometheus_test.go

@@ -41,7 +41,7 @@ func TestPrometheusGeneratesMetrics(t *testing.T) {
 
 	var acc testutil.Accumulator
 
-	err := p.Gather(&acc)
+	err := acc.GatherError(p.Gather)
 	require.NoError(t, err)
 
 	assert.True(t, acc.HasFloatField("go_gc_duration_seconds", "count"))