Settings: Env override support for dynamic settings (#21439)

* Settings: supports env overrrides for dynamic settings

* Settings: makes it possible to explicitly get env override support for dynamic settings

* Make linter happy

pkg/setting/dynamic_settings_test.go

@@ -0,0 +1,35 @@
+package setting
+
+import (
+	"os"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestDynamicSettingsSupport_Override(t *testing.T) {
+	cfg := NewCfg()
+
+	envKey := "GF_FOO_BAR"
+	sectionName := "foo"
+	keyName := "bar"
+	expected := "dynamic value"
+
+	os.Setenv(envKey, expected)
+	defer func() { os.Unsetenv(envKey) }()
+
+	value := cfg.SectionWithEnvOverrides(sectionName).Key(keyName).MustString("default value")
+	require.Equal(t, expected, value)
+}
+
+func TestDynamicSettingsSupport_NoOverride(t *testing.T) {
+	cfg := NewCfg()
+	sectionName := "foo"
+	keyName := "bar"
+	expected := "default value"
+
+	_, err := cfg.Raw.Section(sectionName).NewKey(keyName, expected)
+	require.NoError(t, err)
+	value := cfg.SectionWithEnvOverrides(sectionName).Key(keyName).String()
+	require.Equal(t, expected, value)
+}