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.

etc/telegraf.conf

@@ -52,6 +52,11 @@
   ## 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
@@ -75,9 +80,6 @@
   urls = ["http://localhost:8086"] # required
   ## The target database for metrics (telegraf will create it if not exists).
   database = "telegraf" # required
-  ## Precision of writes, valid values are "ns", "us" (or "µs"), "ms", "s", "m", "h".
-  ## note: using "s" precision greatly improves InfluxDB compression.
-  precision = "s"
 
   ## Retention policy to write to.
   retention_policy = "default"