fix(agent): Fix buffer directory config and document (#15661)

config/config.go

@@ -1513,8 +1513,10 @@ func (c *Config) buildOutput(name string, tbl *ast.Table) (*models.OutputConfig,
 		return nil, err
 	}
 	oc := &models.OutputConfig{
-		Name:   name,
-		Filter: filter,
+		Name:            name,
+		Filter:          filter,
+		BufferStrategy:  c.Agent.BufferStrategy,
+		BufferDirectory: c.Agent.BufferDirectory,
 	}
 
 	// TODO: support FieldPass/FieldDrop on outputs
@@ -1529,8 +1531,6 @@ func (c *Config) buildOutput(name string, tbl *ast.Table) (*models.OutputConfig,
 	c.getFieldString(tbl, "name_suffix", &oc.NameSuffix)
 	c.getFieldString(tbl, "name_prefix", &oc.NamePrefix)
 	c.getFieldString(tbl, "startup_error_behavior", &oc.StartupErrorBehavior)
-	c.getFieldString(tbl, "buffer_strategy", &oc.BufferStrategy)
-	c.getFieldString(tbl, "buffer_directory", &oc.BufferDirectory)
 
 	if c.hasErrs() {
 		return nil, c.firstErr()
@@ -1549,6 +1549,7 @@ func (c *Config) missingTomlField(_ reflect.Type, key string) error {
 	switch key {
 	// General options to ignore
 	case "alias", "always_include_local_tags",
+		"buffer_strategy", "buffer_directory",
 		"collection_jitter", "collection_offset",
 		"data_format", "delay", "drop", "drop_original",
 		"fielddrop", "fieldexclude", "fieldinclude", "fieldpass", "flush_interval", "flush_jitter",

docs/CONFIGURATION.md

@@ -356,7 +356,12 @@ The agent table configures Telegraf and the defaults used across all plugins.
   The type of buffer to use for telegraf output plugins. Supported modes are
   `memory`, the default and original buffer type, and `disk`, an experimental
   disk-backed buffer which will serialize all metrics to disk as needed to
-  improve data durability and reduce the chance for data loss.
+  improve data durability and reduce the chance for data loss. This is only
+  supported at the agent level.
+
+- **buffer_directory**:
+  The directory to use when in `disk` buffer mode. Each output plugin will make
+  another subdirectory in this directory with the output plugin's name.
 
 ## Plugins
 

models/running_output.go

@@ -347,7 +347,11 @@ func (r *RunningOutput) writeMetrics(metrics []telegraf.Metric) error {
 
 func (r *RunningOutput) LogBufferStatus() {
 	nBuffer := r.buffer.Len()
-	r.log.Debugf("Buffer fullness: %d / %d metrics", nBuffer, r.MetricBufferLimit)
+	if r.Config.BufferStrategy == "disk" {
+		r.log.Debugf("Buffer fullness: %d metrics", nBuffer)
+	} else {
+		r.log.Debugf("Buffer fullness: %d / %d metrics", nBuffer, r.MetricBufferLimit)
+	}
 }
 
 func (r *RunningOutput) Log() telegraf.Logger {