Refactor exporter into modules

openclaw_usage_exporter/api.py

@@ -0,0 +1,13 @@
+from __future__ import annotations
+
+from fastapi import FastAPI
+from fastapi.responses import PlainTextResponse
+
+from .metrics import render_metrics
+
+app = FastAPI(title="openclaw-usage-exporter")
+
+
+@app.get("/metrics", response_class=PlainTextResponse)
+def metrics() -> str:
+    return render_metrics()