Add Giga fetch/serve and BlockDB prune metrics (CON-438)

[wen-coding]

Summary

• Add tendermint_autobahn_blockstore_prune_watermark at the BlockDB prune choke point (actual floor after clamp, seeded on open).
• Add tendermint_p2p_giga_fetch{resource,reason} for GetBlock, FullCommitQC, AppQC, CommitQC, and lane-proposal client paths (ok, unavailable, open_stream, send, receive, decode, process, wrong_lane).
• Add tendermint_p2p_giga_serve{resource,reason} on GetBlock empty/error replies (pruned, not_found, store_error) so a lagging node's unavailable can be joined with a tip node's pruned.

Test plan

• GOWORK=off go test --count=0 ./sei-tendermint/autobahn/blockstore ./sei-tendermint/internal/p2p/giga ./sei-tendermint/internal/autobahn/data
• scripts/ramtest.sh ./sei-tendermint/autobahn/blockstore/... ./sei-tendermint/internal/p2p/giga/... ./sei-tendermint/internal/autobahn/data/...
• On a giga testnet node, scrape Prometheus and confirm prune_watermark, fetch, and serve series appear; compare a lagging node's fetch{resource="block",reason="unavailable"} with a tip node's serve{resource="block",reason="pruned"} and next_block{stage="receive"}.

Made with Cursor

[github-actions]

The latest Buf updates on your PR. Results from workflow Buf / buf (pull_request).

Build	Format	Lint	Breaking	Updated (UTC)
✅ passed	✅ passed	✅ passed	✅ passed	Sep 24, 2026, 11:12 PM

[codecov]

Codecov Report

❌ Patch coverage is 69.33333% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.39%. Comparing base (c1f6409) to head (a081928).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
sei-tendermint/internal/p2p/giga/data.go	46.15%	14 Missing ⚠️
sei-tendermint/internal/p2p/giga/avail.go	30.76%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4329      +/-   ##
==========================================
- Coverage   67.49%   66.39%   -1.11%     
==========================================
  Files        2181     2063     -118     
  Lines      167844   155883   -11961     
==========================================
- Hits       113294   103500    -9794     
+ Misses      54540    52373    -2167     
  Partials       10       10              

Flag	Coverage Δ
sei-chain-pr	79.20% <69.33%> (?)
sei-db	74.50% <ø> (ø)
sei-db-state-db	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
sei-tendermint/autobahn/blockstore/metrics.gen.go	100.00% <100.00%> (ø)
sei-tendermint/autobahn/blockstore/store.go	87.50% <100.00%> (+0.10%)	⬆️
sei-tendermint/internal/p2p/giga/metrics.gen.go	100.00% <100.00%> (ø)
sei-tendermint/internal/p2p/giga/metrics.go	100.00% <100.00%> (ø)
sei-tendermint/internal/p2p/giga/avail.go	69.79% <30.76%> (-3.74%)	⬇️
sei-tendermint/internal/p2p/giga/data.go	71.15% <46.15%> (-5.19%)	⬇️

... and 121 files with indirect coverage changes

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[seidroid]

This PR adds metrics only: a BlockDB prune-watermark gauge, updated when the store opens and inside PruneBefore, plus Giga fetch/serve counters whose label sets are fixed and small. Behaviour doesn't change, and I found no correctness issues; the one note is that the new metrics have no tests.

Findings: 0 blocking | 1 non-blocking | 0 posted inline

Blockers

• None at the file/PR level.

Non-blocking

• [suggestion] No tests cover the new metrics. A small test would pin the reason labels and stop them drifting. It could check that PruneBefore sets prune_watermark to the clamped floor, and that serverGetBlock increments serve{block,pruned} and serve{block,not_found} for ErrPruned and ErrNotFound. Any gatherer or registry helper works for reading the values back.

[seidroid]

Successfully created backport PR for giga-1:

• Backport giga-1: Add Giga fetch/serve and BlockDB prune metrics (CON-438) #4331