Add precision rounding to accumulator

Adding precision rounding to the accumulator. This means that now every
input metric will get rounded at collection, rather than at write (and
only for the influxdb output).

This feature is disabled for service inputs, because service inputs
should be in control of their own timestamps & precisions.

agent/agent.go

@@ -118,6 +118,8 @@ func (a *Agent) gatherer(
 
 		acc := NewAccumulator(input.Config, metricC)
 		acc.SetDebug(a.Config.Agent.Debug)
+		acc.SetPrecision(a.Config.Agent.Precision.Duration,
+			a.Config.Agent.Interval.Duration)
 		acc.setDefaultTags(a.Config.Tags)
 
 		internal.RandomSleep(a.Config.Agent.CollectionJitter.Duration, shutdown)
@@ -201,6 +203,8 @@ func (a *Agent) Test() error {
 	for _, input := range a.Config.Inputs {
 		acc := NewAccumulator(input.Config, metricC)
 		acc.SetTrace(true)
+		acc.SetPrecision(a.Config.Agent.Precision.Duration,
+			a.Config.Agent.Interval.Duration)
 		acc.setDefaultTags(a.Config.Tags)
 
 		fmt.Printf("* Plugin: %s, Collection 1\n", input.Name)
@@ -289,6 +293,9 @@ func (a *Agent) Run(shutdown chan struct{}) error {
 		case telegraf.ServiceInput:
 			acc := NewAccumulator(input.Config, metricC)
 			acc.SetDebug(a.Config.Agent.Debug)
+			// Service input plugins should set their own precision of their
+			// metrics.
+			acc.DisablePrecision()
 			acc.setDefaultTags(a.Config.Tags)
 			if err := p.Start(acc); err != nil {
 				log.Printf("Service for input %s failed to start, exiting\n%s\n",