Start first aggregator period at startup time (#3050)

Fixes issue where metrics collected immediately after startup would not
be aggregated.

agent/agent.go

@@ -364,6 +364,8 @@ func (a *Agent) Run(shutdown chan struct{}) error {
 	metricC := make(chan telegraf.Metric, 100)
 	aggC := make(chan telegraf.Metric, 100)
 
+	now := time.Now()
+
 	// Start all ServicePlugins
 	for _, input := range a.Config.Inputs {
 		input.SetDefaultTags(a.Config.Tags)
@@ -404,7 +406,7 @@ func (a *Agent) Run(shutdown chan struct{}) error {
 			acc := NewAccumulator(agg, aggC)
 			acc.SetPrecision(a.Config.Agent.Precision.Duration,
 				a.Config.Agent.Interval.Duration)
-			agg.Run(acc, shutdown)
+			agg.Run(acc, now, shutdown)
 		}(aggregator)
 	}
 

internal/models/running_aggregator.go

@@ -114,6 +114,7 @@ func (r *RunningAggregator) reset() {
 // for period ticks to tell it when to push and reset the aggregator.
 func (r *RunningAggregator) Run(
 	acc telegraf.Accumulator,
+	now time.Time,
 	shutdown chan struct{},
 ) {
 	// The start of the period is truncated to the nearest second.
@@ -132,7 +133,6 @@ func (r *RunningAggregator) Run(
 	// 2nd interval: 00:10 - 00:20.5
 	// etc.
 	//
-	now := time.Now()
 	r.periodStart = now.Truncate(time.Second)
 	truncation := now.Sub(r.periodStart)
 	r.periodEnd = r.periodStart.Add(r.Config.Period)

internal/models/running_aggregator_test.go

@@ -24,7 +24,7 @@ func TestAdd(t *testing.T) {
 	})
 	assert.NoError(t, ra.Config.Filter.Compile())
 	acc := testutil.Accumulator{}
-	go ra.Run(&acc, make(chan struct{}))
+	go ra.Run(&acc, time.Now(), make(chan struct{}))
 
 	m := ra.MakeMetric(
 		"RITest",
@@ -55,7 +55,7 @@ func TestAddMetricsOutsideCurrentPeriod(t *testing.T) {
 	})
 	assert.NoError(t, ra.Config.Filter.Compile())
 	acc := testutil.Accumulator{}
-	go ra.Run(&acc, make(chan struct{}))
+	go ra.Run(&acc, time.Now(), make(chan struct{}))
 
 	// metric before current period
 	m := ra.MakeMetric(
@@ -113,7 +113,7 @@ func TestAddAndPushOnePeriod(t *testing.T) {
 	wg.Add(1)
 	go func() {
 		defer wg.Done()
-		ra.Run(&acc, shutdown)
+		ra.Run(&acc, time.Now(), shutdown)
 	}()
 
 	m := ra.MakeMetric(