feat(sync): networkHead callback metric

[renaynay]

As per request

celestiaorg/celestia-node#2921 (comment)

While this isn't super high priority - it is true that totalSynced metric does not correlate to what the current networkHead is. There is no way currently to determine what the netHead of the node is via metric (local head does not suffice as it's only reporting store head).

[codecov-commenter]

Codecov Report

Attention: 19 lines in your changes are missing coverage. Please review.

Comparison is base (28ff21c) 63.60% compared to head (b6fd46e) 63.41%.

Files	Patch %	Lines
sync/metrics.go	17.39%	19 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #131      +/-   ##
==========================================
- Coverage   63.60%   63.41%   -0.20%     
==========================================
  Files          39       39              
  Lines        3366     3389      +23     
==========================================
+ Hits         2141     2149       +8     
- Misses       1060     1075      +15     
  Partials      165      165              

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[Wondertan]

I think we will have to close this one in favour of another incoming syncer metric PR

[renaynay]

is networkHead added as a metric there? @Wondertan

[Wondertan]

There will be a subjective head(or just head) that will represent the current network head. On the code, we need to differentiate between subjective and network, but on metrics, the subjective will be the network head as it's applied almost instantly. So there will be head and the state syncing or not, which will be enough for the requested purpose