config: set gogc default value when config body is empty (#16052)

* fix: set gogc default value when config body is empty

Signed-off-by: Danial Eskandari <doneskandari@gmail.com>

* refactor: explicitly check value 75 in `TestGoGCDefaultValueOnEmptyConfigBody`

add GoGC assertion in `TestEmptyConfig`, also removed the no longer needed runtime config assignment in `TestEmptyGlobalBlock`

Signed-off-by: Danial Eskandari <doneskandari@gmail.com>

* refactor: remove `TestGoGCDefaultValueOnEmptyConfigBody` to reduce duplicate assertions

Signed-off-by: Danial Eskandari <doneskandari@gmail.com>

---------

Signed-off-by: Danial Eskandari <doneskandari@gmail.com>

config/config.go

@@ -156,6 +156,7 @@ var (
 	// DefaultConfig is the default top-level configuration.
 	DefaultConfig = Config{
 		GlobalConfig: DefaultGlobalConfig,
+		Runtime:      DefaultRuntimeConfig,
 	}
 
 	// DefaultGlobalConfig is the default global configuration.

config/config_test.go

@@ -2222,6 +2222,7 @@ func TestEmptyConfig(t *testing.T) {
 	exp := DefaultConfig
 	exp.loaded = true
 	require.Equal(t, exp, *c)
+	require.Equal(t, 75, c.Runtime.GoGC)
 }
 
 func TestExpandExternalLabels(t *testing.T) {
@@ -2269,7 +2270,6 @@ func TestEmptyGlobalBlock(t *testing.T) {
 	c, err := Load("global:\n", promslog.NewNopLogger())
 	require.NoError(t, err)
 	exp := DefaultConfig
-	exp.Runtime = DefaultRuntimeConfig
 	exp.loaded = true
 	require.Equal(t, exp, *c)
 }