feat(das): add das_sampling_head_lag metric

[operagxsasha]

Partially addresses #3273.

This adds a das_sampling_head_lag gauge that shows how many headers the sampled chain head is behind the network head (NetworkHead - SampledChainHead, never below 0). It reuses the stats the metrics callback already reads, so there's no extra cost. I also added a small unit test for the calculation.

I left out the time-based lag from the issue, since it needs the header timestamp at SampledChainHead. Happy to add it here or in a follow-up, whichever you prefer.

[greptile-apps]

Confidence Score: 5/5

[Low risk] Adds a new observability metric to the data availability sampler.

No outstanding findings block merging.

Summary

This PR adds das_sampling_head_lag from the stats already read by the metrics callback. Since the last review, the metric-collection test was removed; the calculation test remains.

_{Reviews (3) · Last reviewed commit: "test(das): drop head lag collection test"}

[renaynay]

Pls drop TestHeadLagMetricIsCollected (which is failing repeated runs) bc TestHeadLagOf already covers the only new logic

[codecov]

Codecov Report

❌ Patch coverage is 40.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 37.49%. Comparing base (42addd6) to head (8ff32fb).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
das/metrics.go	40.00%	6 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5307      +/-   ##
==========================================
- Coverage   37.52%   37.49%   -0.03%     
==========================================
  Files         321      321              
  Lines       22323    22332       +9     
==========================================
- Hits         8376     8374       -2     
- Misses      12922    12929       +7     
- Partials     1025     1029       +4     

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[operagxsasha]

Pls drop TestHeadLagMetricIsCollected (which is failing repeated runs) bc TestHeadLagOf already covers the only new logic

done