feat: update dependencies and improve caching mechanism

- Added Prometheus client library for metrics collection.
- Refactored garbage collection strategy from random deletion to LRU (Least Recently Used) deletion.
- Introduced per-key locking in cache to prevent race conditions.
- Enhanced logging with structured log messages for cache hits and misses.
- Implemented a retry mechanism for upstream requests with exponential backoff.
- Updated Go modules and indirect dependencies for better compatibility and performance.
- Removed unused sync filesystem implementation.
- Added version initialization to ensure a default version string.

version/version.go

@@ -1,3 +1,9 @@
 package version
 
 var Version string
+
+func init() {
+	if Version == "" {
+		Version = "0.0.0-dev"
+	}
+}