feat(v2): implement ReadIndex for linearizable reads

[marcsanmi]

This PR implements the ReadIndex in the Metastore to ensure linearizable reads, as described in the Raft consensus algorithm. The implementation leverages the Hashicorp Raft library's existing features while adding necessary checks to guarantee read consistency.

Key changes:

1. Update ReadIndex method to verify leadership and return current commit index.
2. Added readIndex method to ensure the node is up-to-date before serving reads.
3. Modified QueryMetadata and GetProfileStats to use readIndex before querying data.
4. Updated CheckReady method to use readIndex for consistency checks.

Raft Algorithm Considerations:

• Hashicorp's Raft implementation already sends a no-op log entry when runLeader is called for the leader to be up to the latest commit index, which aligns with the Raft paper's recommendation for ensuring a committed entry from the current term (thanks @kolesnikovae for the heads up).

Testing Considerations:

• Unit tests were not added for these changes as they primarily interact with the Raft package itself.
• Integration tests would be more suitable for verifying the correct interaction with Raft and ensuring linearizable reads.
• The current metastore lacks integration tests, so adding them would be a significant change that deserves its own focused effort.

[kolesnikovae]

Thanks for taking care of this @marcsanmi!

@korniltsev could you please take a look as well? Just to make 100% sure the approach is valid

[kolesnikovae]

I've reverted the min ready check:

1. This is not quite the same as k8s minReadySeconds
2. It might be useful in deployments outside of k8s

[korniltsev]

LGTM