Refactor running_output buffering

closes #1087

internal/config/config.go

@@ -93,9 +93,15 @@ type AgentConfig struct {
 	// ie, a jitter of 5s and interval 10s means flushes will happen every 10-15s
 	FlushJitter internal.Duration
 
+	// MetricBatchSize is the maximum number of metrics that is wrote to an
+	// output plugin in one call.
+	MetricBatchSize int
+
 	// MetricBufferLimit is the max number of metrics that each output plugin
 	// will cache. The buffer is cleared when a successful write occurs. When
-	// full, the oldest metrics will be overwritten.
+	// full, the oldest metrics will be overwritten. This number should be a
+	// multiple of MetricBatchSize. Due to current implementation, this could
+	// not be less than 2 times MetricBatchSize.
 	MetricBufferLimit int
 
 	// FlushBufferWhenFull tells Telegraf to flush the metric buffer whenever
@@ -182,9 +188,13 @@ var header = `# Telegraf Configuration
   ## ie, if interval="10s" then always collect on :00, :10, :20, etc.
   round_interval = true
 
+  ## Telegraf will send metrics to output in batch of at
+  ## most metric_batch_size metrics.
+  metric_batch_size = 1000
   ## Telegraf will cache metric_buffer_limit metrics for each output, and will
-  ## flush this buffer on a successful write.
-  metric_buffer_limit = 1000
+  ## flush this buffer on a successful write. This should be a multiple of
+  ## metric_batch_size and could not be less than 2 times metric_batch_size
+  metric_buffer_limit = 10000
   ## Flush the buffer whenever full, regardless of flush_interval.
   flush_buffer_when_full = true
 
@@ -526,6 +536,9 @@ func (c *Config) addOutput(name string, table *ast.Table) error {
 	}
 
 	ro := internal_models.NewRunningOutput(name, output, outputConfig)
+	if c.Agent.MetricBatchSize > 0 {
+		ro.MetricBatchSize = c.Agent.MetricBatchSize
+	}
 	if c.Agent.MetricBufferLimit > 0 {
 		ro.MetricBufferLimit = c.Agent.MetricBufferLimit
 	}