fix issue with loading processor config from execd (influxdata#8274)

Author: ssoroka

plugins/common/shim/config.go

@@ -116,7 +116,11 @@ func createPluginsWithTomlConfig(md toml.MetaData, conf config) (loadedConfig, e
 		plugin := creator()
 		if len(primitives) > 0 {
 			primitive := primitives[0]
-			if err := md.PrimitiveDecode(primitive, plugin); err != nil {
+			var p telegraf.PluginDescriber = plugin
+			if processor, ok := plugin.(unwrappable); ok {
+				p = processor.Unwrap()
+			}
+			if err := md.PrimitiveDecode(primitive, p); err != nil {
 				return loadedConf, err
 			}
 		}
@@ -169,3 +173,7 @@ func DefaultImportedPlugins() (config, error) {
 	}
 	return conf, nil
 }
+
+type unwrappable interface {
+	Unwrap() telegraf.Processor
+}

plugins/common/shim/config_test.go

@@ -8,6 +8,7 @@ import (
 	"github.com/influxdata/telegraf"
 	tgConfig "github.com/influxdata/telegraf/config"
 	"github.com/influxdata/telegraf/plugins/inputs"
+	"github.com/influxdata/telegraf/plugins/processors"
 	"github.com/stretchr/testify/require"
 )
 
@@ -55,6 +56,19 @@ func TestLoadingSpecialTypes(t *testing.T) {
 	require.EqualValues(t, 3*1000*1000, inp.Size)
 }
 
+func TestLoadingProcessorWithConfig(t *testing.T) {
+	proc := &testConfigProcessor{}
+	processors.Add("test_config_load", func() telegraf.Processor {
+		return proc
+	})
+
+	c := "./testdata/processor.conf"
+	_, err := LoadConfig(&c)
+	require.NoError(t, err)
+
+	require.EqualValues(t, "yep", proc.Loaded)
+}
+
 type testDurationInput struct {
 	Duration tgConfig.Duration `toml:"duration"`
 	Size     tgConfig.Size     `toml:"size"`
@@ -70,3 +84,18 @@ func (i *testDurationInput) Description() string {
 func (i *testDurationInput) Gather(acc telegraf.Accumulator) error {
 	return nil
 }
+
+type testConfigProcessor struct {
+	Loaded string `toml:"loaded"`
+}
+
+func (p *testConfigProcessor) SampleConfig() string {
+	return ""
+}
+
+func (p *testConfigProcessor) Description() string {
+	return ""
+}
+func (p *testConfigProcessor) Apply(metrics ...telegraf.Metric) []telegraf.Metric {
+	return metrics
+}

plugins/common/shim/testdata/processor.conf

@@ -0,0 +1,2 @@
+[[processors.test_config_load]]
+  loaded = "yep"