Add vector_index_compact_read_bytes metric to track compaction input size

[keisku]

Part of #28578 and follow up on 82e7741#diff-c2003959336de6b22d907014e32e29d09f1d194097b380dd542a91f4d6505680

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Set up a testing env.

./bin/ysqlsh -c "CREATE EXTENSION IF NOT EXISTS vector;"
./bin/ysqlsh -c "CREATE TABLE IF NOT EXISTS items (id int PRIMARY KEY, embedding vector(3));"
./bin/ysqlsh -c "CREATE INDEX IF NOT EXISTS items_embedding_idx ON items USING ybhnsw (embedding vector_l2_ops);"

The metrics are per-tserver, so we need to query the correct node. Find which tserver leads the index tablet.

./build/latest/bin/yb-admin --master_addresses 127.0.0.1:7100,127.0.0.2:7100,127.0.0.3:7100 list_all_masters
Master UUID                             RPC Host/Port           State           Role    Broadcast Host/Port
1e72c027e9d74934bfebcba681588fa0        127.0.0.1:7100          ALIVE           FOLLOWER        N/A
eac81bb7e1fb4bf1bd277978cd1aec35        127.0.0.2:7100          ALIVE           FOLLOWER        N/A
89979012063047b895171a8301ffedcc        127.0.0.3:7100          ALIVE           LEADER  N/A

Verify metrics start at zero.

curl -s http://127.0.0.3:9000/metrics | jq . | grep -A2 vector_index_compact
        "name": "vector_index_compact_write_bytes",
        "value": 0
      },
--
        "name": "vector_index_compact_read_bytes",
        "value": 0
      },

Metrics remain at 0 after INSERT — 10k vectors is well below the 100k flush threshold (vector_index_initial_chunk_size), so no chunks reach disk and compaction never triggers.

./bin/ysqlsh -c "INSERT INTO items SELECT i, ARRAY[random(), random(), random()]::vector FROM generate_series(1, 10000) i;"

curl -s http://127.0.0.3:9000/varz\?raw 2>&1 | grep vector_index_initial_chunk_size
--vector_index_initial_chunk_size=100000

Force vector index compaction.

./build/latest/bin/yb-admin -master_addresses 127.0.0.3:7100 compact_table ysql.yugabyte items ADD_VECTOR_INDEXES
Compacted [yugabyte.items [ysql_schema=public] [000034e10000300080000000000040f9]] tables and associated vector indexes.

grep -hr 'Compaction input' /Users/keisukeumegaki/yugabyte-data/node-3/disk-1/yb-data/tserver/logs
I0218 10:34:03.594872 1884237824 vector_lsm.cc:2472] T 6d235c3f23a54e90889eb0a6475f4126 P 703f5f16dcd340be9142b5a3bf82d9c9 VI 000034e10000300080000000000040fe: Compaction input [chunks: 2, indexes: 1, vectors: 10000]

curl -s http://127.0.0.3:9000/metrics | jq . | grep -A2 vector_index_compact
        "name": "vector_index_compact_write_bytes",
        "value": 840357
      },
--
        "name": "vector_index_compact_read_bytes",
        "value": 841451
      },

---

Note

Low Risk
Adds a new counter and increments it during compaction without changing compaction behavior or persistence logic; main risk is metric miscounting due to relying on chunk file_size().

Overview
Adds a new per-tserver metric, vector_index_compact_read_bytes, to track total input bytes read during Vector LSM compactions.

VectorLSM::DoCompact now sums file_size() across the compaction scope and increments the new counter alongside existing compact_write_bytes, and unit tests are updated/extended to assert the read metric stays at 0 when no compaction occurs and increases appropriately for both background and manual compactions.

^{Written by Cursor Bugbot for commit 9a18f30. This will update automatically on new commits. Configure here.}

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[keisku]

@spolitov Any chance you could take a look at this PR?

[keisku]

@hari90 Any chance you could take a look at this PR?

[hari90]

@keisku sorry for the delay