feat(server/otel): restructure observability metrics and add active sessions gauge

- Moved RateLimitMetrics import path to a more centralized location.
- Introduced a new file for active sessions gauge to track user sessions in the database.
- Updated index.ts to include new metrics and ensure proper initialization of observability metrics.
- Modified various routes and services to utilize the new observability structure.
- Added smoke tests for HTTP and WebSocket metrics to ensure proper metric registration and functionality.
- Enhanced error handling for metrics reading failures to improve observability.

apps/server/tsconfig.json

@@ -21,6 +21,7 @@
   },
   "include": [
     "src/**/*.ts",
-    "src/**/*.d.ts"
+    "src/**/*.d.ts",
+    "instrumentation.ts"
   ]
 }