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.

internal/config/config.go

@@ -77,6 +77,14 @@ type AgentConfig struct {
 	//     ie, if Interval=10s then always collect on :00, :10, :20, etc.
 	RoundInterval bool
 
+	// By default, precision will be set to the same timestamp order as the
+	// collection interval, with the maximum being 1s.
+	//   ie, when interval = "10s", precision will be "1s"
+	//       when interval = "250ms", precision will be "1ms"
+	// Precision will NOT be used for service inputs. It is up to each individual
+	// service input to set the timestamp at the appropriate precision.
+	Precision internal.Duration
+
 	// CollectionJitter is used to jitter the collection by a random amount.
 	// Each plugin will sleep for a random time within jitter before collecting.
 	// This can be used to avoid many plugins querying things like sysfs at the
@@ -108,11 +116,10 @@ type AgentConfig struct {
 	// does _not_ deactivate FlushInterval.
 	FlushBufferWhenFull bool
 
-	// TODO(cam): Remove UTC and Precision parameters, they are no longer
+	// TODO(cam): Remove UTC and parameter, they are no longer
 	// valid for the agent config. Leaving them here for now for backwards-
 	// compatability
-	UTC       bool `toml:"utc"`
-	Precision string
+	UTC bool `toml:"utc"`
 
 	// Debug is the option for running in debug mode
 	Debug bool
@@ -209,6 +216,11 @@ var header = `# Telegraf Configuration
   ## ie, a jitter of 5s and interval 10s means flushes will happen every 10-15s
   flush_jitter = "0s"
 
+  ## By default, precision will be set to the same timestamp order as the
+  ## collection interval, with the maximum being 1s.
+  ## Precision will NOT be used for service inputs, such as logparser and statsd.
+  ## Valid values are "Nns", "Nus" (or "Nµs"), "Nms", "Ns".
+  precision = ""
   ## Run telegraf in debug mode
   debug = false
   ## Run telegraf in quiet mode