A0-4591: Added major sync metric back (although by different name)

[Marcin-Radecki]

Description

This PR brings back major syncing metric that was unwillingly removed in 14 release. It copies some code from polkadot-sdk (metric definition), copy is required since we want different name and MajorSyncingGauge constructor is private in polkadot-sdk. Other than that implementation approach is taken from the polkadot-sdk - the AtomicBool is passed from its creation down to when it's used in the metric gauge.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Testing

I run locally ./scripts/run_nodes.sh -v 4, then immediately run `watch -n -1 ' curl localhost:9616/metrics | grep major_sync' is one console and in the other

[12:22] marol-Latitude-5521:aleph-node (A0-4591 *%) | tail -f run-nodes-local/node-1.log | grep "major sync"
2025-01-24 12:22:36.317  INFO tokio-runtime-worker aleph-block-sync: Switched to major sync state.
2025-01-24 12:22:46.320  INFO tokio-runtime-worker aleph-block-sync: No longer in major sync state.

and I saw correlation when the log appeared, the metric switched its state accordingly.

[timorleph]

This can probably be made quite a bit simpler, see the single comment. But other than that it's alright, so if you think my suggestion is bad feel free to merge.