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/accumulator.go

@@ -17,6 +17,7 @@ func NewAccumulator(
 	acc := accumulator{}
 	acc.metrics = metrics
 	acc.inputConfig = inputConfig
+	acc.precision = time.Nanosecond
 	return &acc
 }
 
@@ -32,6 +33,8 @@ type accumulator struct {
 	inputConfig *internal_models.InputConfig
 
 	prefix string
+
+	precision time.Duration
 }
 
 func (ac *accumulator) Add(
@@ -141,6 +144,7 @@ func (ac *accumulator) AddFields(
 	} else {
 		timestamp = time.Now()
 	}
+	timestamp = timestamp.Round(ac.precision)
 
 	if ac.prefix != "" {
 		measurement = ac.prefix + measurement
@@ -173,6 +177,31 @@ func (ac *accumulator) SetTrace(trace bool) {
 	ac.trace = trace
 }
 
+// SetPrecision takes two time.Duration objects. If the first is non-zero,
+// it sets that as the precision. Otherwise, it takes the second argument
+// as the order of time that the metrics should be rounded to, with the
+// maximum being 1s.
+func (ac *accumulator) SetPrecision(precision, interval time.Duration) {
+	if precision > 0 {
+		ac.precision = precision
+		return
+	}
+	switch {
+	case interval >= time.Second:
+		ac.precision = time.Second
+	case interval >= time.Millisecond:
+		ac.precision = time.Millisecond
+	case interval >= time.Microsecond:
+		ac.precision = time.Microsecond
+	default:
+		ac.precision = time.Nanosecond
+	}
+}
+
+func (ac *accumulator) DisablePrecision() {
+	ac.precision = time.Nanosecond
+}
+
 func (ac *accumulator) setDefaultTags(tags map[string]string) {
 	ac.defaultTags = tags
 }