Add metrics for monitoring client sync process

[KatyaRyazantseva]

Description

We need to add metrics to monitor the client syncing process across different implementations.

Proposed Metric

• lean_node_sync_status (Gauge) - Updates on sync status change with the following labels: not_started | syncing | synced | stalled.

Challenges

Different client implementations may require different approaches, making it difficult to define status labels that work universally across all clients.

Open Questions

Concerns raised by @kamilsa in #16:

Not clear what is the difference between synced, syncing, and stalled. E.g. if we are on the top of the chain are we synced or syncing (waiting for a new block). Also how do we define stalled?

If by sync we mean initial sync after the node is started, then it all makes more sense, but we need to write somewhere that this is initial sync.

TODOs

1. Monitor initial client sync implementations across different clients
2. Define clear criteria for each sync status that works across all implementations
3. Specify whether this metric tracks initial sync only or ongoing synchronization
4. Establish clear definitions for edge cases (e.g., synced vs. waiting for next block, criteria for "stalled")