refactor(obs): migrate metrics runtime/schema and tighten migration guards (#2584)

crates/obs/src/metrics/schema/entry/namespace.rs

@@ -0,0 +1,28 @@
+// Copyright 2024 RustFS Team
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+/// The metric namespace, which represents the top-level grouping of the metric
+#[derive(Debug, Clone, PartialEq, Eq)]
+pub enum MetricNamespace {
+    RustFS,
+}
+
+impl MetricNamespace {
+    #[allow(dead_code)]
+    pub fn as_str(&self) -> &'static str {
+        match self {
+            Self::RustFS => "rustfs",
+        }
+    }
+}