Replace all uses of the `internaltest` package in `sdk/metric`

[MrAlias]

opentelemetry-go/sdk/metric/config_test.go

Lines 133 to 137 in b98dd5e

	store, err := ottest.SetEnvVariables(map[string]string{
	envVarResourceAttributes: "key=value,rk5=7",
	})
	require.NoError(t, err)
	defer func() { require.NoError(t, store.Restore()) }()

Replace with:

t.Setenv(envVarResourceAttributes, "key=value,rk5=7")

[MrAlias]

Assigned to @zhengkezhou1