Design a Thread-Safe Shared Counter

Design and implement a per-key counter. Requirements: - Provide an API such as `incrementAndGet(key)`. - Each call increments the count for that key and returns how many times that key has been called so far. - Start with a simple in-process implementation. - Make the implementation thread-safe. - Compare multiple thread-safety approaches and their trade-offs. - Extend the design to support multiple independent applications or processes running on the same host that all need to share the same counter values.