Add internal function for telegraf version (#4828)

internal/internal.go

@@ -24,13 +24,32 @@ var (
 	TimeoutErr = errors.New("Command timed out.")
 
 	NotImplementedError = errors.New("not implemented yet")
+
+	VersionAlreadySetError = errors.New("version has already been set")
 )
 
+// Set via the main module
+var version string
+
 // Duration just wraps time.Duration
 type Duration struct {
 	Duration time.Duration
 }
 
+// SetVersion sets the telegraf agent version
+func SetVersion(v string) error {
+	if version != "" {
+		return VersionAlreadySetError
+	}
+	version = v
+	return nil
+}
+
+// Version returns the telegraf agent version
+func Version() string {
+	return version
+}
+
 // UnmarshalTOML parses the duration from the TOML config file
 func (d *Duration) UnmarshalTOML(b []byte) error {
 	var err error

internal/internal_test.go

@@ -182,3 +182,15 @@ func TestCompressWithGzip(t *testing.T) {
 
 	assert.Equal(t, testData, string(output))
 }
+
+func TestVersionAlreadySet(t *testing.T) {
+	err := SetVersion("foo")
+	assert.Nil(t, err)
+
+	err = SetVersion("bar")
+
+	assert.NotNil(t, err)
+	assert.IsType(t, VersionAlreadySetError, err)
+
+	assert.Equal(t, "foo", Version())
+}